drm/i915: Limit MST to <= 8bpc once again

My attempt at allowing MST to use the higher color depths has
regressed some configurations. Apparently people have setups
where all MST streams will fit into the DP link with 8bpc but
won't fit with higher color depths.

What we really should be doing is reducing the bpc for all the
streams on the same link until they start to fit. But that requires
a bit more work, so in the meantime let's revert back closer to
the old behavior and limit MST to at most 8bpc.

Cc: stable@vger.kernel.org
Cc: Lyude Paul <lyude@redhat.com>
Tested-by: Geoffrey Bennett <gmux22@gmail.com>
Fixes: f1477219

 ("drm/i915: Remove the 8bpc shackles from DP MST")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111505
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190828102059.2512-1-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>
(cherry picked from commit 75427b2a

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>