drm/vc4: Correct logic on stopping an HVS channel

When factoring out __vc4_hvs_stop_channel, the logic got inverted from
	if (condition)
	  // stop channel
to
	if (condition)
	  goto out
	//stop channel
	out:
and also changed the exact register writes used to stop the channel.

Correct the logic so that the channel is actually stopped, and revert
to the original register writes.

Fixes: 6d01a106 ("drm/vc4: crtc: Move HVS init and close to a function")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>