drm/vc4: crtc: Clear the PixelValve FIFO during configuration

drm/vc4: crtc: Clear the PixelValve FIFO during configuration

Even though it's not really clear why we need to flush the PV FIFO during
the configuration even though we started by flushing it, experience shows
that without it we get a stale pixel stuck in the FIFO between the HVS and
the PV.

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/ccd6269ba37b2f849ba6e62471c99bd93a4548a0.1599120059.git-series.maxime@cerno.tech