opencl: route large q6_K lm_head to the flat GEMV (#26427)
* add a direct size condition for `large` weights; the original
dimension condition is insufficient -- q6_K lm_head for gemma-4 E2B
has [1536, 262144], which is big enough to slowdown gemv_noshuffle but
does not satisfy the dimension condition (ne0 >= 2048)
