r600g: fix up depth write swizzles.

r600g: fix up depth write swizzles.

For some reason r600c, emits extra instructions in the FP to do the depth write output swizzle,
I'm not sure this is required, so here I'm doing it in the exports.

this fixes the mesa trivial demos tri-depthwrite and tri-depthwrite2, it doesn't fix
the glsl1 gl_FragDepth writing test however.