drm/i915: Fix hdmi clock readout with pixel repeat

Account for the pixel multiplier when reading out the HDMI
mode dotclock. Makes the state checked happier on my ILK when using
double clocked modes.

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>