drm/ast: Replace module-init boiler-plate code with DRM helpers

Remove custom ast_init() and ast_exit() functions and initialize the
module with DRM module helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222082831.196562-3-javierm@redhat.com