drm: move quirk_addfb_prefer_xbgr_30bpp handling to drm_driver_legacy_fb_format too

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180921134704.12826-7-kraxel@redhat.com