i965: Set max texture buffer size to hardware limit

i965: Set max texture buffer size to hardware limit

Previously we were leaving this at the default of 64K, which meets the
spec but is too small for some real uses. The hardware can handle up to
128M.

User was complaining about this on freenode ##OpenGL today.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>