radeonsi: add Polaris12 support (v3)

radeonsi: add Polaris12 support (v3)

v2: use gfxip names for llvm 4.0+
v3: use tonga for llvm <= 3.8, drop gfxip name,
we can just change that we change the other asics.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>