nv50,nvc0: add X8Z24_UNORM, fix stencil-only formats

S8_UINT will become useful when ARB_texture_stencil8 becomes supported by
mesa. The other stencil formats are needed for ARB_stencil_texturing.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
mesa/st: implement ARB_stencil_texturing

If StencilSampling is enabled on the texture object, pass in an
equivalent stencil-only format.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50,nvc0: leave queries on during blit, turn them on for 2d engine

Fixes the new logic of the conditional rendering piglit test.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2 10.1" <mesa-stable@lists.freedesktop.org>