drm/ast: Allocate HW cursor BOs during cursor-plane initialization

The BOs are eventually released by the cursor plane's destroy callback.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-5-tzimmermann@suse.de