drm/ast: Remove last traces of struct ast_gem_object

The ast driver has switched to struct drm_vram_gem_object a while ago.
This patch removes a function and forward declaration that were forgotten
before.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-2-tzimmermann@suse.de