gallium/u_threaded_context: Use PIPE_MAX_SHADER_SAMPLER_VIEWS for sampler_buffers

gallium/u_threaded_context: Use PIPE_MAX_SHADER_SAMPLER_VIEWS for sampler_buffers

Sampler views and samplers may not be the same limit; in fact one is 32
while the other is 128.  The sampler_buffers field is tracking sampler
views (yes, naming is confusing) so we should use the right limit.

Fixes: e9c41b321488 ("gallium/u_threaded: add buffer lists - tracking of buffers referenced by tc")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15988>