drm/i915: Document that we implement WaIncreaseLatencyIPCEnabled

Display w/a #1141 is also known as WaIncreaseLatencyIPCEnabled.
Add that to the comment.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190503173807.10834-1-ville.syrjala@linux.intel.com