anv/descriptor_set: Initialize immutable_samplers to NULL
anv/descriptor_set: Initialize immutable_samplers to NULL

Previously this wasn't a problem.  However, with the new API update,
descriptor sets can now be sparse so the client doesn't have to provide an
entry for every binding.  This means that it's possible for a binding to be
uninitialized other than the memset.  In that case, we want to have a null
array of immutable samplers.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler