drm/msm: Don't implicit-sync if only a single ring

If there is only a single ring (no-preemption), everything is FIFO order
and there is no need to implicit-sync.

Mesa should probably just always use MSM_SUBMIT_NO_IMPLICIT, as behavior
is undefined when fences are not used to synchronize buffer usage across
contexts (which is the only case where multiple different priority rings
could come into play).

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>