drm/amdgpu: split gfx8 gpu init into sw and hw parts

Calculate the driver state in sw_init and program the
registers in hw init.

Acked-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>