drm/i915: Clean up HDMI deep color handling a bit

Reogranize the HDMI deep color state computation to just
loop over possible bpc values. Avoids having to maintain
so many variants of the clock etc.

The current code also looks confused w.r.t. port_clock vs.
bw_constrained. It would happily update port_clock for
deep color but then not actually enable deep color due to
bw_constrained being set. The new logic handles that case
correctly.

v2: Pull stuff into separate funcs (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190828183424.7856-1-ville.syrjala@linux.intel.com