[mlir][sparse][gpu] only support default algorithm for SpGEMM

Rationale:
This is the approach taken for all the others too (SpMV, SpMM, SDDMM),
so it is more consistent to follow the same path (until we have a need
for more algorithms). Also, in a follow up revision, this will allow
us to remove some unused GEMM ops.

Reviewed By: K-Wu

Differential Revision: https://reviews.llvm.org/D157542