Allow DP MST to output any color depth. This means deep color as
well as falling back to 6bpc if we would otherwise require too
much bandwidth.

TODO: We should probably extend bw_contstrained scheme to force
all streams on the link to 6bpc if we can't fit the new stream(s)
otherwise.

v2: Use a proper for-loop (Jani)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190326142556.21176-3-ville.syrjala@linux.intel.com