ruby,gpu-compute: bugfix for GPU_VIPER* protocols

ruby,gpu-compute: bugfix for GPU_VIPER* protocols

12db50c895 changed how directory mapping works, but it seems to have
broken the VIPER variants of the GPU protocols. The fix involves
declaring the function in the related '.sm' files.

Change-Id: I116980d42a4aa648369058b529c9f8d9693eb894
Reviewed-on: https://gem5-review.googlesource.com/8521
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>