freedreno: avoid stalling at ringbuffer wraparound

freedreno: avoid stalling at ringbuffer wraparound

Because of how the tiling works, we can't really flush at arbitrary
points very easily.  So wraparound is handled by resetting to top of
ringbuffer.  Previously this would stall until current rendering is
complete.  Instead cycle through multiple ringbuffers to avoid a stall.

Signed-off-by: Rob Clark <robclark@freedesktop.org>