drm/amd/pm: do not expose the smu_context structure used internally in power

This can cover the power implementation details. And as what did for
powerplay framework, we hook the smu_context to adev->powerplay.pp_handle.

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>