agx: Bump preamble_storage_size to 512
nir_opt_preamble is now aware of the internal uniforms we insert, so it can use the whole uniform file available to it. This lets us push more (all?) uniform loads in Dolphin ubershaders to the preamble. Signed-off-by:Alyssa Rosenzweig <alyssa@rosenzweig.io> Part-of: <!20562>