drm/amd/powerplay: update is_sw_smu_xgmi check

Add check for is_sw_smu routine and drop check
for amdgpu_dpm which seems non-sense.

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