drm/amd/powerplay: cover the powerplay implementation details V3

This can save users much troubles. As they do not
actually need to care whether swSMU or traditional
powerplay routine should be used.

V2: apply the fixes to vi.c and cik.c also
V3: squash in oops fix

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