cpu: x64: brgemm: fix assertion due to invalid EVEX offset

[rjoursler]

Fixes the following assertion due to offset calculated here on GPU large buffer tests on SPR systems by disabling brgemm for large buffers.

[rjoursle@sdpdnn630225 ~]$ ~/dnnl/build_debug/tests/benchdnn/benchdnn --mode-modifier=P --ip --engine=gpu --dt=bf16:bf16:f32 --stag=ab --dtag=ab mb21ic13oc133326203
terminate called after throwing an instance of 'Xbyak::Error'
  what():  offset is too big

Fixes MFDNN-13161.

[rjoursler]

make test