drm/amdgpu: fix dma_fence_wait without reference

[ Upstream commit 42068e1e ]

We need to grab a reference to the fence we wait for.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>