drm/amdgpu: fix the eviction fence dereference

The dma_resv_add_fence() already refers to the added fence.
So when attaching the evciton fence to the gem bo, it needn't
refer to it anymore.

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>