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