drm/vc4: crtc: Move the CRTC initialisation to a separate function

The upcoming patches to turn the TXP into a full-blown CRTC will have the
same CRTC initialisation code, so let's move it into a separate, public,
function so that we can reuse it later on.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/3a3026c0e7408895d154d8dea454cf6d1c459715.1591882579.git-series.maxime@cerno.tech