drm/amd/pm: do not expose implementation details to other blocks out of power

Those implementation details(whether swsmu supported, some ppt_funcs supported,
accessing internal statistics ...)should be kept internally. It's not a good
practice and even error prone to expose implementation details.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>