drm/i915: Document that PPT==CPT and WPT==LPT

For our purposes PPT is equivalent to CPT, and WPT is equivalent to
LPT. Document that fact.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620130310.13245-4-ville.syrjala@linux.intel.com


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