drm/ast: Wrap cursor framebuffer access in drm_gem_fb_begin/end_cpu_access()

drm/ast: Wrap cursor framebuffer access in drm_gem_fb_begin/end_cpu_access()

Call drm_gem_fb_begin_cpu_access() and drm_gem_fb_end_cpu_access()
around cursor image updates. Imported buffers might have to be
synchronized for CPU access before they can be used.

Ignore errors from drm_gem_fb_begin_cpu_access(). These errors can
often be transitory. The cursor image will be updated on the next
frame. Meanwhile display a white square where the cursor would be.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>>
Link: https://patch.msgid.link/20251126094626.41985-4-tzimmermann@suse.de