Add dedicated helper to convert from XRGB8888 to ARGB2101010. Sets
all alpha bits to make pixels fully opaque.

v2:
	* set correct format in struct drm_framebuffer (Javier)
	* 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/20230102112927.26565-9-tzimmermann@suse.de