drm/i915/skl: Provide skl-specific pll hw state cross-checking

v2: rebase on top of the hw state flattening.

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