drm/amdgpu: fix double reference dropping

The reference to object fence is dropped at the end of the loop.
However, it is dropped again outside the loop. The reference can be
dropped immediately after calling dma_fence_wait() in the loop and
thus the dropping operation outside the loop can be removed.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>