vc4: Fix scaling W projection of the Z coordinate when there's a Z offset.
vc4: Fix scaling W projection of the Z coordinate when there's a Z offset.

Fixes piglit glsl-fs-fragcoord-zw-perspective, es3conform
gl_FragCoord_z_frag, and the rest of the piglit glsl 1.10 interpolation
tests.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler