drm/vc4: hvs: Make the stop_channel function public

During the transition from the firmware to the KMS driver, we need to pay
particular attention to how we deal with the pixelvalves that have already
been enabled, otherwise either timeouts or stuck pixels can occur. We'll
thus need to call the function to stop an HVS channel at boot.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/a9d5f0891c3bc1deb6b16d56ca6994ed912ec7c7.1599120059.git-series.maxime@cerno.tech