drm/i915/fbc: Streamline frontbuffer busy bits handling

If the frontbuffer bits say this fbc instance isn't affected just
skip the whole thing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-3-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>