drm/vc4: crtc: Only access the PixelValve registers if we have to

The CRTC hooks are called both for the TXP and the pixelvalve, yet some
will read / write the registers as if the device was a pixelvalve, which
won't really work.

Let's make sure we only access those registers if we are running on a
PixelValve.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/b55e31869304c748920c261eba87b3275dbeb297.1591882579.git-series.maxime@cerno.tech