vc4: Kill a bunch of color write calculation when colormask is all off.

vc4: Kill a bunch of color write calculation when colormask is all off.

I could have done this in the bit that generates the ANDs and ORs, but
it's probably generally useful.  Sadly, I still need this even if I move
to NIR, because I can't yet express my read of the destination color in
NIR, which I would need to move my blend/logicop/colormask handling into
NIR.

total uniforms in shared programs: 13497 -> 13455 (-0.31%)
uniforms in affected programs:     101 -> 59 (-41.58%)
total instructions in shared programs: 40797 -> 40296 (-1.23%)
instructions in affected programs:     1639 -> 1138 (-30.57%)