i965: Add a driconf option to disable flush throttling.

Normally when submitting the first batch buffer after a flush, we
check whether the GPU has completed processing of the first batch
buffer of the previous frame.  If it hasn't, we wait for it to finish
before submitting any more batches.  This prevents GPU-heavy and
CPU-light applications from racing too far ahead of the current frame,
but at the expense of possibly lower frame rates.  Sometimes when
benchmarking we want to disable this mechanism.

This patch adds the driconf option "disable_throttling" to disable the
throttling mechanism.