vc4: Add support for depth clears and tests within a tile.

vc4: Add support for depth clears and tests within a tile.

This doesn't load/store the Z contents across submits yet.  It also
disables early Z, since it's going to require tracking of Z functions
across multiple state updates to track the early Z direction and whether
it can be used.

v2: Move the key setup to before the search for the key.