drm/amdgpu: sanitize fence numbers

Looks like the VCE block sometimes still sends nonsense
fence numbers on startup.

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