drm/i915: Simplify hdmi_12bpc_possible()

With the output_types bitmask there's no need to loop through
the encoders anymore when checking for HDMI+non-HDMI cloning.

v2: Use output_types bitmask
v3: Fix the logic to really check that there are no non-HDMI encoders

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v2)
Link: http://patchwork.freedesktop.org/patch/msgid/1466621833-5054-11-git-send-email-ville.syrjala@linux.intel.com