drm/i915: Stop consulting plane->fb

We want to get rid of plane->fb on atomic drivers. Stop looking at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-10-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>