drm/format-helper: Add drm_fb_xrgb8888_to_rgb888()

Add XRGB8888 emulation support for devices that can only do RGB888.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929191201.34456-4-noralf@tronnes.org