drm/i915/power: convert {SKL, ICL}_PW_CTL_IDX_TO_PG() macros to a function

Add pw_ctl_idx_to_pg() helper function to deduplicate the open-coded
usage of the {SKL,ICL}_PW_CTL_IDX_TO_PG() macros.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/3aa74825db0b900f93b94aa89d0242a354929b85.1750855148.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>