drm/ast: Move cursor update code to ast_show_cursor()

A call to ast's show-cursor function now receives the cursor image
and updates the buffer. The change splits off image update and
base-address update into separate functions.

v3:
	* move ast_{show,hide}_cursor() in a previous patch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927090309.10254-4-tzimmermann@suse.de