intel: Stop creating the wrapped stencil irb.

intel: Stop creating the wrapped stencil irb.

There were only two places it was really used at this point, which was
in the batchbuffer emit of the separate stencil packets for gen6/7.
Just write in the ->stencil_mt reference in those two places and ditch
all this flailing around with allocation and refcounts.

v2: Fix separate stencil on gen7.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>