drm/amdkfd: evict svm bo worker handle error

Migrate vram to ram may fail to find the vma if process is exiting
and vma is removed, evict svm bo worker sets prange->svm_bo to NULL
and warn svm_bo ref count != 1 only if migrating vram to ram
successfully.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>