vulkan: Use mul_mat_vec_id for small values of n (llama/18918)

vulkan: Use mul_mat_vec_id for small values of n (llama/18918)

Change ggml_vk_mul_mat_vec_id_q_f16 to loop over the batch dimension and
update the indexing calculations in get_offsets.

Mat-vec is faster than mat-mat for small values of n. We don't get the same
reuse of the weights as in the non-ID path, but with this the cost is linear
in n rather than n>1 being far slower than n==1.