drm/i915: Reject HDMI 12bpc if the sink doesn't indicate support

drm/i915: Reject HDMI 12bpc if the sink doesn't indicate support

Check that the sink really declared 12bpc support before we enable it.
This should not actually never happen since it's mandatory for HDMI
sinks to support 12bpc if they support any deep color modes. But
reality disagrees with the theory and there are actually sinks in
the wild that violate the spec.

v2: Fix the output_types check
    Update commit message to state that these things are in fact real

Cc: stable@vger.kernel.org
Cc: Nicholas Sielicki <nicholas.sielicki@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99250
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213175818.24958-1-ville.syrjala@linux.intel.com
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
(cherry picked from commit c750bdd3e7e204cc88b32806c3864487a03cd84b)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>