i965: Disentangle batch and state buffer flushing.

i965: Disentangle batch and state buffer flushing.

We now flush the batch when either the batchbuffer or statebuffer
reaches the original intended batch size, instead of when the sum of
the two reaches a certain size (which makes no sense now that they're
separate buffers).

With this change, we also need to update our "are we near the end?"
estimate to require separate batch and state buffer space.  I obtained
these estimates by looking at the size of draw calls in the Unreal 4
Elemental Demo (using INTEL_DEBUG=flush and always_flush_batch=true).

This will significantly impact the size of our batches.  I've adjusted
both down to try and be roughly similar to what we had been doing.  On
various benchmarks, a 20kB batch and 16kB statebuffer seemed to about
right, but we may need to adjust this further.  I tried a 16kB batch,
but that regressed Synmark OglMultithread performance by a fair bit.
32kB for both would have significantly increased our batch sizes.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>