drm/ast: Swap framebuffer writes on big-endian machines

drm/ast: Swap framebuffer writes on big-endian machines

Swap the pixel data when writing to framebuffer memory on big-endian
machines. Fixes incorrect output. Aspeed graphics does not appear to
support big-endian framebuffers after AST2400, although the feature
has been documented.

There's a lengthy discussion at [1].

v5:
- avoid restricted cast from __be16 (kernel test robot)

Signed-off-by: René Rebe <rene@exactco.de>
Link: https://lore.kernel.org/dri-devel/20251202.170626.2134482663677806825.rene@exactco.de/
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20251212.210504.1355099120650239629.rene@exactco.de