drm/i915: State readout support for WRPLLs

Still tacked onto the side, but slowly getting there.

v2: Don't forget the debugfs file.

v3 (from Paulo): Don't forget to check the power domains.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>