drm/amdgpu/smu11: add a helper to set the power source

Add a common smu11 helper to set the AC/DC power source.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/1043


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