drm: add drm_fb_xrgb8888_to_rgb888_dstclip()

Simliar to drm_fb_xrgb8888_to_rgb565_dstclip() but converts to rgb888
instead of rgb565.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190405095219.9231-5-kraxel@redhat.com