drm/i915: Add max_bpc property for DP MST

Allow the user to limit the output bpc with DP MST.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326142556.21176-4-ville.syrjala@linux.intel.com