drm/vc4: Add monochrome mode to the VEC.

The VEC supports not producing colour bursts for monochrome output.
It also has an option for disabling the chroma input to remove
chroma from the signal.

Now that there is a DRM_MODE_TV_MODE_xx defined for monochrome,
plumb this in.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>