i965: Always use Y-tiled buffers on SKL+

i965: Always use Y-tiled buffers on SKL+

Starting with Skylake, the display engine is capable of scanning out from
Y-tiled buffers. As such, we can and should use Y-tiling for better efficiency.
This also has the added benefit of being able to fast clear the winsys buffer.

Note that the buffer allocation done for mipmaps will already never allocate an
X-tiled buffer for GEN9.

This has an almost universal positive impact on benchmarks, some improving by as
much as 20%.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>