nv50,nvc0: add support for stencil sampling from depth_stencil textures

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
mesa/st: pass stencil sampling flag along in pipe_sampler_view

Also enable ARB_stencil_texturing when PIPE_CAP_STENCIL_SAMPLING is set.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50,nvc0: add ability to texture S8_UINT

This will become useful when ARB_texture_stencil8 becomes supported by
mesa.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50/ir: allow load propagation when flags are defined

The old condition disallowed load propagation any time flags were
defined, even with e.g. set. The new condition disallows it only with
immediate propagation.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>