drm/ttm: add gem base object

Add drm_gem_object struct to ttm_buffer_object, so ttm objects are a gdm
object superclass.  Add a function to check whenever a given bo actually
uses the embedded drm_gem_object.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-2-kraxel@redhat.com