drm/amd/powerplay: remove useless soft pptable in Asic related backend

The soft pptable was used for re-uploading pptable as cache, but since
previous commits, the generic codes for uploading pptable are used and
backend is released during resetting powerplay. So it becomes redundance.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>