drm/i915: Balance prepare_fb/cleanup_fb

drm/i915: Balance prepare_fb/cleanup_fb

intel_prepare_plane_fb() bails early if there is no fb (or rather
no obj, which is the same thing). intel_cleanup_plane_fb() does not.
This means the steps performed by intel_cleanup_plane_fb() aren't
balanced with with what was done intel_prepare_plane_fb() if there
is no fb for the plane. These hooks get called for every plane in
the state regardless of whether they have an fb or not.

Add a matching null obj check to intel_cleanup_plane_fb() to restore
the balance.

Note that intel_cleanup_plane_fb() has sufficient protections
already in place that the imbalance doesn't cause any real problems.
But having things be in balance seems nicer anyway, and might help
avoid some surprises in the future.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110183228.8199-5-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>