vc4: Add support for two-sided color.

vc4: Add support for two-sided color.

It's fairly easy, thanks to Rob Clark's lowering code.  Fixes
two-sided-lighting and 4 vertex-program-two-side testcases, while
regressing 8 testcases that involve enabling two-sided color while only
initializing one of the two colors in the VS.  If you're enabling two
sided color, it's of course expected that you really do set up both
colors, so this is still an improvement (and when we set up a linker for
TGSI, we'll hopefully fix those 8 fails).