radv: Use VRAM for upload buffers if entire VRAM is CPU-visible.

radv: Use VRAM for upload buffers if entire VRAM is CPU-visible.

Not doing this for APUs because spilling is quite likely, due to
overall VRAM pressure.

Also adding a flag to disable for performance debugging.

Finally adds some memset for places where we depended on the memory
being initialized to zero, which we won't get with VRAM anymore.
(I think these places should stop depending on it since it hides
 issues with executing the cmdbuffer multiple times, but this
  preserves behavior)

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7979>