pan/midgard: Switch sources to an array for trinary sources

pan/midgard: Switch sources to an array for trinary sources

We need three independent sources to support indirect SSBO writes (as
well as textures with both LOD/bias and offsets). Now is a good time to
make sources just an array so we don't have to rewrite a ton of code if
we ever needed a fourth source for some reason.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>