drm/format-helper: Add conversion from XRGB8888 to BGR888

Add XRGB8888 emulation helper for devices that only support BGR888.

Signed-off-by: Kerem Karabay <kekrby@gmail.com>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/9A67EA95-9BC7-4D56-8F87-05EAC1C166AD@live.com