drm/i915/skl: Add GEN8_L3SQCREG4 to HW whitelist

drm/i915/skl: Add GEN8_L3SQCREG4 to HW whitelist

Required for WaDisableLSQCROPERFforOCL:skl

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 changes (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/1453412634-29238-7-git-send-email-arun.siluvery@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>