drm/i915: hw state readout support for fdi m/n

We want to use the fdi m/n values to easily compute the adjusted mode
dotclock on pch ports. Hence make sure the values stored in the pipe
config are always reliable.

v2: Fixup FDI TU readout.

v3: Rebase on top of moved cpu_transcoder.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>