drm/i915/fbc: Track plane visibility

Currently the code (ab)uses cache->vma to indicate the plane
visibility. I want to nuke that so let's add a dedicated boolean
for this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-7-ville.syrjala@linux.intel.com


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