drm/amdkfd: Fix mm reference in SVM eviction worker

Use the mm reference from the fence. This allows removing the
svm_bo->svms pointer, which was problematic because we cannot assume
that the struct kfd_process containing the svms is still allocated
without holding a refcount on the process.

Use mmget_not_zero to ensure the mm is still valid, and drop the svm_bo
reference if it isn't.

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