drm/amd/powerplay: revise reading/writing pptable on Polaris10

Change the way we store pptables in the driver to better
facilitate eventual runtime updates for debugging.

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>