drm/vc4: Calculate bpc based on max_requested_bpc

This aligns vc4 with Intel, AMD and Synopsis drivers and fixes max bpc
connector property not working as expected on monitors with YCbCr 4:2:2
support but not deep color support.

max_bpc in connector state is clamped at max_bpc from display info and
the latter only takes deep color modes into account so it will always
be 8, even if the display can do 4:2:2 12-bit output.

Signed-off-by: Matthias Reichl <hias@horus.com>