drm/vc4: hvs: Make sure our channel is reset

In order to clear our intermediate FIFOs that might end up with a stale
pixel, let's make sure our FIFO channel is reset every time our channel is
setup.

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: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b34c562b36177c758dd2e9d84bceb07689bfbe05.1599120059.git-series.maxime@cerno.tech