i965: Prefer Y-tiling on Gen6+.

i965: Prefer Y-tiling on Gen6+.

In the past, we preferred X-tiling for color buffers because our BLT
code couldn't handle Y-tiling.  However, the BLT paths have been largely
replaced by BLORP on Gen6+, which can handle any kind of tiling.

We hadn't measured any performance improvement in the past, but that's
probably because compressed textures were all untiled anyway.

Improves performance in GLB27_TRex_C24Z16_FixedTime by 7.69231%.

v2: Rebase on top of Eric's untiled-for-larger-than-aperture changes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>