GS/HW: Better avoid target uploads on HW clear

[stenzek]

Description of Changes

Noticed COD was uploading its depth buffer that it just cleared, which was all zeros.

Previous heuristic made sure it was the previous draw that cleared it, to skip the upload. But when you draw other stuff inbetween the clear and the first draw that uses the target (like cod does with its skybox), that doesn't work.

So, make the hw_clear a flag instead, so that later partial writes don't get lost to full-target writes.

Rationale behind Changes

Fewer target uploads. Vroom vroom

Suggested Testing Steps

Smoke test, check some affected games like NFSU, God Hand, God of War.