gpu_group.c: check_stride: extract out set_stride
gpu_group.c: check_stride: extract out set_stride

That is, isolate the part that looks for a stride
such that it can easily be replaced.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/ppcg,Meinersbur/ppcg