drm/ast: Embed struct drm_device in struct ast_private

Turns struct ast_private into a subclass of struct drm_device by
embedding the latter. This allows for using DRM's managed device
allocation.

The use of struct drm_device.dev_private is deprecated. The patch
converts the last remaining users to to_ast_private().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-7-tzimmermann@suse.de