i965: Increase the number of array texture levels to the hardware limit.

i965: Increase the number of array texture levels to the hardware limit.

The EXT_texture_array required only 64, but GL 3.0 required 256.
Since we're already exposing values that can get us way beyond our
ability to map the single object directly, go ahead and expose all the
way to hardware limits.

Tested with new piglit EXT_texture_array/maxlayers on gen7.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>