i965: Make the mt->target of multisample renderbuffers be 2D_MS.

i965: Make the mt->target of multisample renderbuffers be 2D_MS.

Mostly mt->target == 2D_MS just results in a few checks that we don't try
to allocate multiple LODs and don't try to do slice copies with them.  But
with the introduction of binding renderbuffers to textures, we need more
consistency.

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