radeonsi: merge constant and shader buffers descriptor lists into one

radeonsi: merge constant and shader buffers descriptor lists into one

Constant buffers: slot[16], .. slot[31] (ascending)
Shader buffers: slot[15], .. slot[0] (descending)

The idea is that if we have 4 constant buffers and 2 shader buffers, we only
have to upload 6 slots. That optimization is left for a later commit.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>