drm/vc4: crtc: Move the CRTC disable out

We'll need to reuse the part that disables the HVS and PixelValve during
boot too, so let's create a separate 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: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/5288fb72ed2da643085dce1bc7f6d6f656bf176e.1599120059.git-series.maxime@cerno.tech