nvc0: increase texture buffer object alignment to 256 for pre-GM107

nvc0: increase texture buffer object alignment to 256 for pre-GM107

We currently don't pass the low byte of the address via the surface
info, so in order to work with images, these have to implicitly be
aligned to 256. The proprietary driver also doesn't go out of its way to
provide lower alignment.

Fixes GL45-CTS.texture_buffer.texture_buffer_texture_buffer_range

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>