drm/vc4: Skip ULPS latching when we're in that ULPS state already.

It seems that trying to go from unlatched to unlatched will time out
waiting for STOP, and we can just skip that.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171031193258.17373-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>