freedreno/drm: Avoid unitialized timestamp in submit fail

Saw a flood of "waiting on invalid fence" with a completely bogus looking fence # in a log of a rather strange low-memory crash. Not sure if it is coming from memory corruption in userspace, but if a submit ioctl is failing due to failed allocation (or other reason) we would get left with random stack garbage as the fence #. Let's not have that as a potential problem.

Signed-off-by: Rob Clark robdclark@chromium.org