vulkan: MOE aware mat_mul_id tile selection (#29182)

mut_mul_id selected its matmul tile with total token count.
For MoE dispatch grid the true N per workgroup is per-expert rows.
At pp128 on Sarvam 30B that is 6, not 128, so the picker took the l-tile for ~6 live rows.
Most workers in each group had nothing to do.
This wasted time. The slow part was 55% of the whole job.