drm/i915/fbc: Extract intel_fbc_update()

Pull the fbc enable vs. disable stuff into a small helper so
we don't have to have it pollute the higher level modeset code.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702204603.596-3-ville.syrjala@linux.intel.com