intel: Add support for the new flush_with_flags extension.

intel: Add support for the new flush_with_flags extension.

This gives us more information about why we're flushing that we can
use for handling our throttling.

v2 (Kenneth Graunke): Rebase on latest master, add missing
   FLUSH_VERTICES and FLUSH_CURRENT, which fixes a regression in Glean's
   polygonOffset test.
v3 (anholt): Drop FLUSH_CURRENT -- FLUSH_VERTICES is what we need, which
   is "get any queued prims out of VBO and into the driver", not "update
   ctx->Current so we can read it with the CPU."  Also drop batch->used
   check, which intel_batchbuffer_flush() does anyway.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>