drm/ast: Remove cursor double buffering

drm/ast: Remove cursor double buffering

Update the cursor image via damage handling in-place. The cursor's
double buffering has no visible effect on the output, so remove it.

Done in preparation of switching ast to GEM SHMEM helpers. Removing
double buffering will allow us to use the same data structure for
primary and cursor plane.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013112923.769-5-tzimmermann@suse.de