drm/vc4: hdmi: Add a function to retrieve the CSC matrix

The CSC matrix to use depends on the output format, its range and the
colorspace.

Since we're going to add more colorspaces, let's move the CSC matrix
retrieval to a function.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20221207-rpi-hdmi-improvements-v3-7-bdd54f66884e@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>