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.

SetTevKonstColor, SetTevColor are only ever called with those bitfields as args, so probably the type of third arg should just be changed so it doesn't require casting all the time to the same thing.