drm/format-helper: Add conversion from XRGB8888 to 15-bit RGB555 formats

Add conversion from XRGB8888 to XRGB1555, ARGB1555 and RGBA5551, which
are the formats currently supported by the simplefb infrastructure. The
new helpers allow the output of XRGB8888 framebuffers to firmware
scanout buffers in one of the 15-bit formats.

v3:
	* use __le* for destination buffers (Jose, kernel test robot)
v2:
	* test 15-bit results with local endianness (Jose)

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/20230102112927.26565-10-tzimmermann@suse.de