drm/amdgpu: fix error handling in amdgpu_cs_process_fence_dep

We always need to drop the ctx reference and should check
for errors first and then dereference the fence pointer.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>