radeonsi: don't load unused compute shader input SGPRs and VGPRs

radeonsi: don't load unused compute shader input SGPRs and VGPRs

Basically, don't load GRID_SIZE or BLOCK_SIZE if they are unused, determine
whether to load BLOCK_ID for each component separately, and set the number
of THREAD_ID VGPRs to load. Now we should get the maximum CS launch wave
rate in most cases.

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