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>