drm/vc4: hdmi: Always try to have the highest bpc

Currently we take the max_bpc property as the bpc value and do not try
anything else.

However, what the other drivers seem to be doing is that they would try
with the highest bpc allowed by the max_bpc property and the hardware
capabilities, test if it results in an acceptable configuration, and if
not decrease the bpc and try again.

Let's use the same logic.

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