drm/i915/bxt: Add GEN8_L3SQCREG4 to HW whitelist

drm/i915/bxt: Add GEN8_L3SQCREG4 to HW whitelist

Required for WaDisableLSQCROPERFforOCL:bxt

According to WA database these are only applicable for BXT:A0 but since
A0 and A1 shares the same GT these are extended for A1 as well.

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/1453412634-29238-6-git-send-email-arun.siluvery@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>