drm/ast: Pin and map cursor source BO during update

The ast driver used to lock the cursor source BO during updates. Locking
should be done internally by the BO's implementation, so we pin it instead
to system memory. The mapping information is also stored in the BO. No
need to have an extra argument to the kmap function.

v2:
	* pin cursor BOs to current location

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-5-tzimmermann@suse.de