drm/format-helper: Store RGB565 in little-endian order

Fix to-RGB565 conversion helpers to store the result in little-
endian byte order. Update test cases as well.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-5-tzimmermann@suse.de