drm/amdgpu/swSMU: set AC/DC mode based on the current system state (v2)

Check of the pointer exists and we are actually on AC power.

v2: fix error message to reflect AC/DC mode.

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>