drm/i915/cnl: Extend WM workaround with IPC for CNL

CNL:A & CNL:B have same workaround as KBL to increase wm level latency
by 4us if IPC is enabled.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817134529.2839-6-mahesh1.kumar@intel.com