drm/amdgpu: Remove dead code in amdgpu_pm.c

As we have unify powergate_uvd/vce/mmhub to set_powergating_by_smu,
and set_powergating_by_smu was supported by both dpm and powerplay.
so remove the else case.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>