radeonsi: clean up and deduplicate code around internal compute dispatches

radeonsi: clean up and deduplicate code around internal compute dispatches

In addition to the cleanup, there are these changes in behavior:
- clear_render_target waits for idle after the dispatch and then flushes
  L0-L1 caches (this was missing)
- sL0 is no longer invalidated before the dispatch, because src resources
  don't use it
- sL0 is no longer invalidated after the dispatch if dst is an image

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4761>