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.

IMO having SetTevKonstColor take an s64 would just make things confusing. I'd like to keep the casts unless you think dropping them is critical.