nvk: Add a new platform abstraction

This MR totally refactors NVK to use a new, fully pluggable platform abstraction. This abstraction doesn't leak any DRM or nouveau.ko details into the driver, allowing us to potentially add winsys implementations to run on other kernels. It also provides a nice cleanup of a number of the interfaces. As a side-effect, the arbitrary limits we had before on bind, signal, and wait counts are now gone. The new code handles arbitrarily many of these.