drm/vc4: Fix spurious GPU resets due to BO reuse.

We were tracking the "where are the head pointers pointing" globally,
so if another job reused the same BOs and execution was at the same
point as last time we checked, we'd stop and trigger a reset even
though the GPU had made progress.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit c4ce60dc)