winsys/radeon: increase the size of buffer cache
winsys/radeon: increase the size of buffer cache

This should fix this performance regression:
https://bugs.freedesktop.org/show_bug.cgi?id=88227

Reviewed-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler