drm/i915/kbl: IPC workaround for kabylake

Display Workarounds #1141
IPC (Isoch Priority Control) may cause underflows.

KBL WA: When IPC is enabled, watermark latency values must be increased
by 4us across all levels. This brings level 0 up to 6us.

Changes since V1:
 - Add Workaround number in commit & code
Changes since V2 (from Paulo):
 - Bikeshed the WA tag so it looks like the others

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201154940.24446-4-mahesh1.kumar@intel.com