drm/amdkfd: Make svm_migrate_put_sys_page static

This function is only used in this source file.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>