This reverts commit 5cb043cf.

While the numbers were impressive for drawoverhead, it comes at the cost
of additional flushes, which for gmem access (what we actually care about
most!) would greatly increase the actual cost to render.  Also, gl_driver2
overhead is increased, probably due to spending time in the kernel for the
flushes.

drawoverhead's win came from the increased flushing causing the GPU to
start processing the buffers sooner on everything but test 1, which
already had some incremental flushing happening.  That was certainly not
intended by the change.

Part-of: <mesa/mesa!11599>