Revert "drm/i915: Disable 12bpc hdmi for now"

HDMI 12bpc should be working fine now. Let it loose.

This reverts commit 5e3daaca

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v2: Rebased due to CHV/BXT port clock check improvemnts

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-and-tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>