drm/i915/fbc: Introduce intel_fbc_add_plane()

In order to better encapsulate the FBC implementation
introduce a small helper to do the plane<->FBC instance
association.

We'll also try to structure the plane init code such
that introducing multiple FBC instances will be easier
down the line.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-13-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>