drm/i915/cnp: Document WaSouthDisplayDisablePWMCGEGating

No functional change since WA is already applied.
But since it has different names on different databases,
let's document it here to avoid future confusion.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306012812.19779-1-rodrigo.vivi@intel.com