drm/i915/icl: add icelake_get_ddi_pll()

Implement the hardware state readout code.

Thanks to Animesh Manna for spotting this problem.

Cc: Animesh Manna <animesh.manna@intel.com>
Credits-to: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522002558.29262-11-paulo.r.zanoni@intel.com