drm/i915/bxt: Add WaEnablePooledEuFor2x6

Pooled EU is enabled by default for BXT but for fused down 2x6 parts it is
advised to turn it off. But there is another HW issue in these parts (fused
down 2x6 parts) before C0 that requires Pooled EU to be enabled as a
workaround. In this case the pool configuration changes depending upon
which subslice is disabled. This doesn't affect if the device has all 3
subslices enabled.

Userspace need to know min no. of eus in a pool as it varies based on which
subslice is disabled, this is not yet exported because userspace support is
not available yet. Once the support is available this needs to be exported
using getparam ioctls.

v2: s/subslice_total/subslice_per_slice as it is a more logical field (Mika)

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Winiarski, Michal <michal.winiarski@intel.com>
Cc: Zou, Nanhai <nanhai.zou@intel.com>
Cc: Yang, Rong R <rong.r.yang@intel.com>
Cc: Tim Gore <tim.gore@intel.com>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>