drm/amd/powerplay: expose the function of smu read argument

Expose the function of smu_read_smc_arg to get argument value from SMU11.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Kevin Wang <Kevin1.Wang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>