drm/vc4: crtc: Move HVS setup code to the HVS driver

The CRTC in vc4 is backed by two devices, the HVS that does the composition
and the PixelValve that does the timing generation.

The writeback is kind of a special case since it doesn't have an associated
pixelvalve but goes straight from the HVS to the TXP. Therefore, it makes
sense to move out the HVS setup code into helpers so that we can also reuse
them from the TXP driver.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/96443394e81429ee38f070cfe231701b07e56d69.1591882579.git-series.maxime@cerno.tech