drm/i915: document PCH_NOP

There's a difference between PCH_NONE and PCH_NOP: the former means we
don't have a PCH while in the latter we do, but it doesn't have the
south display.
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608123330.31003-3-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -632,7 +632,7 @@ enum intel_pch {
 	PCH_KBP,        /* Kaby Lake PCH */
 	PCH_CNP,        /* Cannon Lake PCH */
 	PCH_ICP,	/* Ice Lake PCH */
-	PCH_NOP,
+	PCH_NOP,	/* PCH without south display */
 };
 
 enum intel_sbi_destination {