drm/i915/bxt: Determine BXT slice/subslice/EU info

Modify the Gen9 SSEU info initialization logic to support
Broxton. Broxton reuses the SKL fuse registers but has at most
1 slice and 6 EU per subslice.

Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>