drm/i915/bxt: add DDI port HW readout support

Add support for reading out the HW state for DDI ports. Since the actual
programming is very similar to the CHV/VLV DPIO PLL programming we can
reuse much of the logic from there.

This fixes the state checker failures I saw on my BXT with HDMI output.

v2:
- rebased on v2 of patch 4/5

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>