drm/amd/swSMU: add callback to set AC/DC power source (v2)

This is needed to tell the SMU firmware what state is in
in certain cases.  DC mode does not allow overclocking
for example.

v2: split Evan's original patch (Alex)

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


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