r300g: move emission of the MSPOS regs into the framebuffer state

r300g: move emission of the MSPOS regs into the framebuffer state

Now the question is whether we are allowed to ignore gl_rasterization_rules and
pipe_rasterizer_state::multisample. The former is invariant anyway and
I think the latter would need re-emitting the AA state which is quite costly,
considering that it implicitly flushes the whole pipeline (all AA regs
in the AA state are *unpipelined*).