radv: copy stages instead of serializing NIR for GPL with the RETAIN flag

radv: copy stages instead of serializing NIR for GPL with the RETAIN flag

When the driver gets a cache hit for the binary, we still have to
retain shaders because we can't know if the LTO pipeline will be a
cache hit as well.

Though, serializing the NIR is too costly and most of the libraries
took more than 10ms to be created, which isn't acceptable. To fix this,
keep track of the shaders stage info for libs with the RETAIN flag.

This might be replaced by NIR caching later if it's worth a try.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22327>