drm/amd/pm: do not expose power implementation details to amdgpu_pm.c

amdgpu_pm.c holds all the user sysfs/hwmon interfaces. It's another
client of our power APIs. It's not proper to spike into power
implementation details there.

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>