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

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>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2805>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2805>