drm: reset empty state in transitional helpers

Transitional drivers might not have all the state frobbing lined up
yet. But since the initial code has been merged a lot more state was
added, so we really need this.

Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reported-and-tested-by: John Hunter <zhaojunwang@pku.edu.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>