Draft: panfrost: Dependency tracking + blit fixes

@icecream95 reported several bugs with the native blit logic. 2 of them were indeed introduced by the new blitter (see patch 3 and 4), but pan_blit() also uncovered an issue in the dependency tracking code. This issue is addressed in patch 1 which unfortunately introduces unwanted waits in the submit path. The second patch tries to avoid those waits by declaring a pool of 16 sync objects, so we hopefully don't have to wait when we garbage collect old fences. I honestly don't know if that's the best way to fix the dep tracking code, but I couldn't come with anything better. So please, don't hesitate to suggest alternatives...