drm/i915: Fix HDMI 12bpc and pixel repeat clock readout for DDI platforms

Take the HDMI 12bpc mode and pixel repeat into account when extracting
the dotclock from the hardware on DDI platforms.

Tested on HSW only.

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>