drm/vc4: crtc: Move HVS init and close to a function

In order to make further refactoring easier, let's move the HVS channel
setup / teardown to their own function.

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/fb1b5299d1636ddce8340b51a80d51641839f83b.1599120059.git-series.maxime@cerno.tech