drm/ast: Avoid upcasting to struct ast_device

Several functions receive an instance of struct drm_device only to
upcast it to struct ast_device. Improve type safety by passing the
AST device directly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911115347.899148-11-tzimmermann@suse.de