radeonsi: rework compute scratch buffer

radeonsi: rework compute scratch buffer

Instead of having a scratch buffer per program, have one per
context.

Also removed the per kernel wave count calculations, but
that only helped if the total number of waves in the dispatch
was smaller than sctx->scratch_waves.

v2: Fix style issue.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>