drm/amdgpu: Drop 'fence' check in 'to_amdgpu_amdkfd_fence()'

Return value of container_of(...) can't be null, so null check is not
required for 'fence'. Hence drop its NULL check.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c:93 to_amdgpu_amdkfd_fence() warn: can 'fence' even be NULL?

Cc: Felix Kuehling <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Srinivasan Shanmugam <[e-mail]>
Reviewed-by: Felix Kuehling <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c

···
 		return NULL;
 
 	fence = container_of(f, struct amdgpu_amdkfd_fence, base);
-	if (fence && f->ops == &amdkfd_fence_ops)
+	if (f->ops == &amdkfd_fence_ops)
 		return fence;
 
 	return NULL;