freedreno/drm: fix invalid-cmdstream-size with older kernels

A cmdstream of size zero is invalid. But this can appear in various places where we emit a pointer to state. This doesn't show up with newer kernels (newer than v5.0) which use "softpin", but on earlier kernels can result in:

[drm:msm_ioctl_gem_submit [msm]] ERROR invalid cmdstream size: 0

Since the pointer value doesn't matter in these cases, the easy solution is just to not emit a cmds table entry in this case.

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