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.

Needs an update indeed. I didn't know we still used an explicit state version, so I'll update the PR to bump the version.