drm/amd/powerplay: rewrite pp_sw_init to make code readable

Actually, pp_sw_init executes pptable_init and backend_init orderly if
they are initialized successfully. So rewrite it to make code more
readable.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>