drm/i915/fbc: Move plane pointer into intel_fbc_state

Currently we track the FBC plane as a pointer under intel_fbc
and also as a i9xx_plane_id under intel_fbc_state. Just store
the pointer once in the fbc state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-18-ville.syrjala@linux.intel.com


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