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_MONOCHROME defined, plumb
this in.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619153913.2804051-3-dave.stevenson@raspberrypi.com