drm/i915: Remove the 8bpc shackles from DP MST

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