freedreno: no-op render when we need a fence

freedreno: no-op render when we need a fence

If app tries to create a fence but there is no rendering to submit, we
need a dummy/no-op submit.  Use a string-marker for the purpose.. mostly
since it avoids needing to realize that the packet format changes in
later gen's (so one less place to fixup for a5xx).

Signed-off-by: Rob Clark <robdclark@gmail.com>