drm/i915/icl: add the main CDCLK functions

This commit adds the basic CDCLK functions, but it's still missing
pieces of the display initialization sequence.

v2:
 - Implement the voltage levels.
 - Rebase.
v3:
 - Adjust to the new "bypass" clock (Imre).
 - Call intel_dump_cdclk_state() too.
 - Rename a variable to avoid confusion.
 - Simplify the DVFS part.
v4:
 - Remove wrong bit definition (James).
 - Also drive-by fix the coding style for the register definition we
   touched.
v5:
 - Comment style (checkpatch).

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180206193346.18272-1-paulo.r.zanoni@intel.com