When looking into the Faron Woods fifolog, I noticed this code was quite
high in the profile (~10%). Clearing 4096 entries from the vector isn't
needed every draw, we only need to do this when the cache was actually
valid in the first place.
Should provide a slight general performance boost.
This was causing the depth buffer to be discarded as well, which
has an effect on mobiles (doesn't get loaded into tile memory).
If we find this is hindering performance (remember, the EFB is
only a 640x528 texture), it may be worth changing the interface to
support discarding only the colour buffer.
