drm/atomic-helper: Export both plane and modeset check helpers
The default call sequence for these two parts won't fit for all
drivers. So export the two pieces and explain with a bit of kerneldoc
when each should be called.
v2: Squash in fixup from Rob to actually add the newly exported
functions to headers
Cc: Rob Clark <robdclark@gmail.com> Reviewed-by: Rob Clark <robdclark@gmail.com> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>