drm/amd/powerplay: revise caching the soft pptable and add it's size

This fixes a bug in the pptable access interface that could lead to
a crash.  Check the pointer before using it.

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>