gallium: add support for sampling from stencil of depth_stencil texture

Adds a PIPE_CAP_STENCIL_SAMPLING to indicate support for the feature,
and a stencil_sampling bit in pipe_sampler_view to indicate that the
stencil should be sampled instead of the depth.

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