radeonsi: use a fragment shader blit instead of DB->CB copy for ZS CPU mappings

radeonsi: use a fragment shader blit instead of DB->CB copy for ZS CPU mappings

This mainly removes and simplifies code that is no longer needed.

There were some issues with the DB->CB stencil copy on gfx10, so let's
just use a fragment shader blit for all ZS mappings. It's more reliable.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>