nvc0: increase alignment to 256 for texture buffers on fermi

nvc0: increase alignment to 256 for texture buffers on fermi

When binding as textures, the alignment can be 16. However when binding
as an image, the address has to be aligned to 256. (Also when binding as
an RT, but that can't happen with GL or current gallium APIs.)

Reported-by: Roy Spliet <nouveau@spliet.org>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>