drm/doc: kerneldoc for quirk_addfb_prefer_xbgr_30bpp

Shuts up warning noise.

Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-22-daniel.vetter@ffwll.ch