drm/i915/gen9: Add WaOCLCoherentLineFlush

This is mainly required for future enabling of pre-emptive
command execution.

v2: explain purpose of change (Chris)

Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453412634-29238-9-git-send-email-arun.siluvery@linux.intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>