drm/i915/fbc: Eliminate possible_framebuffer_bits

drm/i915/fbc: Eliminate possible_framebuffer_bits

Not sure what the point of this fbc->possible_frontbuffer_bits is.
And especially don't see why it's returning all the bits when
fbc is not even enabled. So let's just get rid of this and only
say we are interested in the plane's frontbuffer bits when fbc
is actually enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-2-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>