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 changed it to `std::array<int4,4> s_tev_color;`. I didn't go with [4][4] because our ArraySize macro behaves IMO unintuitively for multidimensional arrays (and a nested std::array is just ugly).