drm/ast: Support cursor buffers objects in I/O memory

drm/ast: Support cursor buffers objects in I/O memory

Copy the ARGB4444 cursor buffer to system memory if it is located in
I/O memory. While this cannot happen with ast's native GEM objects, an
imported buffer object might be on the external device's I/O memory.

If the cursor buffer is located in system memory continue to use it
directly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>>
Link: https://patch.msgid.link/20251126094626.41985-3-tzimmermann@suse.de