drm/ast: Update cursor image and checksum from same function

Cursor image and checksum go hand in hand. Update both in the same
place. The helper cannot fail, so remove the return type.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-5-tzimmermann@suse.de