media: v4l docs: move some cross-reference identifiers

media: v4l docs: move some cross-reference identifiers

Sphinx doesn't allow placing those cross-reference identifies
everywhere. Misplacing them cause those warnings:

  Documentation/userspace-api/media/v4l/hist-v4l2.rst:818: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/hist-v4l2.rst:853: WARNING: undefined label: reserved-formats (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst:47: WARNING: undefined label: reserved-formats (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:39: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:53: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:67: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:83: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:97: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/subdev-formats.rst:140: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-decoder-cmd.rst:75: WARNING: undefined label: decoder-cmds (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-dqevent.rst:234: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst:156: WARNING: undefined label: dv-bt-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:41: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:246: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:269: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-modulator.rst:100: WARNING: undefined label: modulator-txsubchans (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-g-tuner.rst:119: WARNING: undefined label: tuner-rxsubchans (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:167: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:245: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:396: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
  Documentation/userspace-api/media/v4l/vidioc-subdev-enum-mbus-code.rst:73: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>