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

Build tested only.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-6-kraxel@redhat.com