drm/vc4: Fix races when the CS reads from render targets.

With the introduction of bin/render pipelining, the previous job may
not be completed when we start binning the next one.  If the previous
job wrote our VBO, IB, or CS textures, then the binning stage might
get stale or uninitialized results.

Fixes the major rendering failure in glmark2 -b terrain.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: ca26d28b ("drm/vc4: improve throughput by pipelining binning and rendering jobs")
Cc: stable@vger.kernel.org