drm/i915/skl: Query DPLL attached to port on SKL

Modify the implementation to query DPLL attached to a SKL port.

v2: Rebase on top of the run-time PM on DPMS series (Damien)

v3: Modified as per review comments from Paulo

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