i965: Move singlesample_mt to the renderbuffer.

i965: Move singlesample_mt to the renderbuffer.

Since only window system renderbuffers can have a singlesample_mt, this
lets us drop a bunch of sanity checking to make sure that we're just a
renderbuffer-like thing.

v2: Fix a badly-written comment (thanks Kenneth!), drop the now trivial
    helper function for set_needs_downsample.

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