VideoCommon: fix indirect texture coordinate computation in TEV.

Even if an indirect texture stage doesn't actually sample a texture, we
still need to emit the relevant math.

Fixes Issue 7092.
VideoCommon: correctly compute whether an indirect texture stage is active.

This is consistent with the implementation in PixelShaderGen.
Merge pull request #344 from magumagu/tev-indirect

VideoCommon: fix indirect texture coordinate computation in TEV.