Add dedicated helper to convert from XRGB8888 to ARGB8888. Sets
all alpha bits to make pixels fully opaque.

v3:
	* use __le32 for destination buffer (Jose, kernel test robot)
v2:
	* use cpubuf_to_le32()
	* type fixes
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-8-tzimmermann@suse.de