drm/i915: Double the port clock when using double clocked modes with 12bpc

Currently we're forgetting to double the port clock when using double
clocked modes with 12bpc on HDMI. We're only accounting for the 1.5x
factor due to the 12bpc. So further double the 1.5x port clock when we
have a double clocked mode.

Unfortunately I don't have any displays that support both 12bpc and
double clocked modes, so I was unable to test this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chandra Konduru <Chandra.konduru@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>