drm/vc4: crtc: Clear the PixelValve FIFO on disable

drm/vc4: crtc: Clear the PixelValve FIFO on disable

In order to avoid a stale pixel getting stuck on mode change or a disable
/ enable cycle, we need to make sure to flush the PV FIFO on disable.

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/26fe48b09d77088679ed0c8cb8cf0db2f108195e.1599120059.git-series.maxime@cerno.tech