drm/vc4: crtc: Turn pixelvalve reset into a function

The driver resets the pixelvalve FIFO in a number of occurences without
always using the same sequence.

Since this will be critical for BCM2711, let's move that sequence to a
function so that we are consistent.

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/fb31003a9eee02c4b949556299ff41f0a113499a.1599120059.git-series.maxime@cerno.tech