Use the Z AOV for the hydra depth buffer

[sebastienblor]

For the hydra depth buffer, we are currently relying on a "P" AOV, and then we use the camera matrix to compute a depth out of it.
We could instead use a "Z" AOV and provide it directly to hydra, as a "linear" depth

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-17104