drm/i915: Stop updating plane->fb/crtc

drm/i915: Stop updating plane->fb/crtc

We want to get rid of plane->fb/crtc on atomic drivers. Stop setting
them.

v2: Fix up the comment in intel_crtc_active() and
    nuke the rest of the stale comments (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #v1
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-9-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>