nv50: access only the available amount of textures

nv50: access only the available amount of textures

The textures array is defined as a number of PIPE_MAX_SAMPLERS per shader stage.
Currently nv50 driver handles only 3 shader stages, thus we wreck chaos when
accessing array-out-of-bounds.

Fixes a segfault in piglit/bin/arb_texture_buffer_object-data-sync -fbo -auto

Cc: 9.1 9.2 10.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>