lib/i915/perf: add support for new EuDualSubslicesTotalCount var

We already assume that the number of subslices is the same as the
number of dualsubslices.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>