llvmpipe: Implement stencil export

llvmpipe: Implement stencil export

Pretty trivial, fixes the issue that we're expected to be able to blit
stencil surfaces (as the blit just relies on util blitter code which needs
stencil export to do it).
2 piglits skip->pass, 11 fail->pass

v2: prettify, keep different stencil ref value handling out of depth/stencil
test itself.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>