drm/vc4: hdmi: Add full range RGB helper

We're going to need to tell whether we want to run with a full or
limited range RGB output in multiple places in the code, so let's create
a helper that will return whether we need with full range or not.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>