Required for WaEnablePreemptionGranularityControlByUMD:skl,bxt

This register is added to HW whitelist to support WA required for future
enabling of pre-emptive command execution, WA implementation will be in
userspace and it cannot program this register if it is not on HW whitelist.

v2: explain purpose of WA (Chris)
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453412634-29238-3-git-send-email-arun.siluvery@linux.intel.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>