st/mesa: add ARB_texture_stencil8 support (v4)

st/mesa: add ARB_texture_stencil8 support (v4)

if we support stencil texturing, enable texture_stencil8
there is no requirement to support native S8 for this,
the texture can be converted to x24s8 fine.

v2: fold fixes from Marek in:
   a) put S8 last in the list
   b) fix renderable to always test for d/s renderable
    fixup the texture case to use a stencil only format
    for picking the format for the texture view.
v3: hit fallback for getteximage
v4: put s8 back in front, it shouldn't get picked now (Ilia)

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>