drm/amd/powerplay: drop sienna_cichlid hardcode of using pptable

Drop the hardcode of sienna_cichlid which will force to use softpptable,
so that it can use pptable on vbios once the value of pp_table_id get
from vbios is 0.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>