intel: Use a CPU map of the batch on LLC-sharing architectures.

intel: Use a CPU map of the batch on LLC-sharing architectures.

Before, we were keeping a CPU-only buffer to accumulate the batchbuffer in,
which was an improvement over mapping the batch through the GTT directly
(since any readback or other failure to stream through write combining
correctly would hurt).  However, on LLC-sharing architectures we can do better
by mapping the batch directly, which reduces the cache footprint of the
application since we no longer have this extra copy of a batchbuffer around.

Improves performance of GLBenchmark 2.1 offscreen on IVB by 3.5% +/- 0.4%
(n=21).  Improves Lightsmark performance by 1.1 +/- 0.1% (n=76).  Improves
cairo-gl performance by 1.9% +/- 1.4% (n=57).

No statistically significant difference in GLB2.1 on SNB (n=37).  Improves
cairo-gl performance by 2.1% +/- 0.1% (n=278).