GPU: Only load the relevant color components upon writes to the tev color registers

The other two components need not be valid upon write, hence loading them results in glitches.

Fixes https://code.google.com/p/dolphin-emu/issues/detail?id=6783.

Also cleaned up the whole mess that surrounded these registers.

I don't think this is needed. This will only happen if two identical BP commands are send. So the second one has no effect.