radeonsi: merge sampler and image descriptor lists into one

radeonsi: merge sampler and image descriptor lists into one

Sampler slots: slot[8], .. slot[39] (ascending)
Image slots: slot[7], .. slot[0] (descending)

Each image occupies 1/2 of each slot, so there are 16 images in total,
therefore the layout is: slot[15], .. slot[0]. (in 1/2 slot increments)

Updating image slot 2n+i (i <= 1) also dirties and re-uploads slot 2n+!i.

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