radv: do not insert shaders in cache when it's disabled

radv: do not insert shaders in cache when it's disabled

When the application doesn't provide its own pipeline cache,
the driver uses a in-memory cache but it shouldn't insert any
entries when the cache is explicitely disabled by the user.

Found while running my experimental pipeline-db tool with a
ton of shaders, the memory footprint was just huge, and sometimes
the process was even killed...

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>