drm/i915/fbc: Nuke fbc.enabled

fbc.enabled == (fbc.crtc != NULL), so let's just nuke fbc.enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-11-ville.syrjala@linux.intel.com


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