drm/i915: implement WaClearTdlStateAckDirtyBits

This is to fix a GPU hang seen with mid thread pre-emption
and pooled EUs.

v2. Use IS_BXT_REVID instead of IS_BROXTON and INTEL_REVID

v3. And use correct type for register addresses

Signed-off-by: Tim Gore <tim.gore@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458571049-854-1-git-send-email-tim.gore@intel.com