drm/amdgpu/display: properly guard the calls to swSMU functions

It's only applicable on newer asics.  We could end up here when
using DC on older asics like SI or KV.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1170


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