drm/i915: Clear old hw.fb & co. from slave plane's state

drm/i915: Clear old hw.fb & co. from slave plane's state

Let's do the intel_plane_copy_uapi_to_hw_state() before we bail out
due to both old and new uapi.crtc being NULL. This will drop the
reference to the old hw.fb for planes that are transitioning from
being a slave plane to simply being disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110183228.8199-2-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>