drm/i915/power: relocate {SKL,ICL}_PW_CTL_IDX_TO_PG()

Move the {SKL,ICL}_PW_CTL_IDX_TO_PG() macros from intel_display_regs.h
to intel_display_power_well.c. The mapping from index to PG can be
hidden there.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/18e40b77eeb3517a056f1e567672163ec568ec55.1750855148.git.jani.nikula@intel.com


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