i965/dri: Handle X-tiled modifier

i965/dri: Handle X-tiled modifier

This doesn't really "do" anything because the default tiling for the
winsys buffer is X tiled. We do however want the X tiled modifier to
work correctly from the API perspective, which would imply that if you
set this modifier, and later do a get_modifier, you get back at least X
tiled.

Running with a modified kmscube, here are the bandwidth measurements.

Linear:
Read bandwidth: 1039.31 MiB/s
Write bandwidth: 1453.56 MiB/s

Y-tiled:
Read bandwidth: 458.29 MiB/s
Write bandwidth: 542.12 MiB/s

X-tiled:
Read bandwidth: 575.01 MiB/s
Write bandwidth: 606.25 MiB/s

Cc: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>