drm/vram: use embedded gem object

Drop drm_gem_object from drm_gem_vram_object, use the
ttm_buffer_object.base instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-3-kraxel@redhat.com