drm/i915: Add broadcast RGB property for DP MST

Add the "Broadcast RGB" property to MST connectors, and implement
the same logic for it as we have in the SST code.

v2: Extract and reuse intel_dp_limited_color_range()

Cc: Ivan Vlk <ari@adyline.sk>
Tested-by: Ivan Vlk <ari@adyline.sk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108821
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/20190326142556.21176-1-ville.syrjala@linux.intel.com