drm/i915: pixel multiplier readout support for pch ports

Now that we painstakingly track the shared pch dplls we can finally
implement pixel mutliplier readout support for pch ports, too.

v2: Undo the temporary hack to disable the sdvo pixel multiplier
cross-checking.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>