drm/amdgpu: Fix null pointer issue in amdgpu_cs_wait_any_fence

The array[first] may be null when the fence has already been signaled.

BUG: SWDEV-136239

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