radeonsi: put image, fmask, and sampler descriptors into one array

radeonsi: put image, fmask, and sampler descriptors into one array

The texture slot is expanded to 16 dwords containing 2 descriptors.
Those can be:
- Image and fmask, or
- Image and sampler state

By carefully choosing the locations, we can put all three into one slot,
with the fmask and sampler state being mutually exclusive.

This improves shaders in 2 ways:
- 2 user SGPRs are unused, shaders can use them as temporary registers now
- each pair of descriptors is always on the same cache line

v2: cosmetic changes: add back v8i32, don't load a sampler state & fmask
    at the same time

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