intel/gen12: Fix memory corruption issues in fused Gen12 parts.

According to the BSpec page for MEDIA_VFE_STATE, on Gen12 platforms "if a fused configuration has fewer threads than the native POR configuration, the scratch space allocation is based on the number of threads in the base native POR configuration". However we currently use the subslice count from devinfo->num_subslices[0], which only includes the subslices currently enabled by the platform fusing. This leads to scratch space underallocation and occasional hangs.

The problem is likely to affect most Gen12 GPUs with less than 96 EUs. GFXBench5 Aztec Ruins is able to reproduce the issue fairly reliably.

Fixes: 9e5ce30d "intel: fix the gen 12 compute shader scratch IDs"