broadcom/vc5: Add lowering for txf_ms to a txf on a 2x2-scaled texture.

broadcom/vc5: Add lowering for txf_ms to a txf on a 2x2-scaled texture.

The HW has no native sampler support for multisample textures, but since
we only need to support txf_ms and the layout is UIF, we just need to
scale up the texcoords and then add in the sample.

This drops the old TEXTURE_MSAA_ADDR special uniform, since we're treating
MSAA textures as textures, rather than basically texbos like VC4 had to.