drm/amdgpu: fix bug in fence driver fini

Using wrong counter for walking fences.  Fixes
a crash when unloading the driver.

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