[XLA:GPU] Set the requested block size for the tiling scheme to 1.

When we refactored the 0-2-1 tiling implementation with a kernel mapping scheme
implementation in cl/221393383, we intended to set the requested block size to
1 to preserve the behavior of the original implementation. We accidently set
the value to 2, which affects the programs with a block size in a normalized
dimension that is not less than 2.

No observable performance impact.

PiperOrigin-RevId: 222142668