radeonsi: flush DB caches only when transitioning from DB to texturing

radeonsi: flush DB caches only when transitioning from DB to texturing

Use the mechanism of si_decompress_textures, but instead of doing
the actual decompression, just flag the DB cache flush there.

This removes a lot of unnecessary DB cache flushes.

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