drm/amd/powerplay: no MGPU fan boost enablement on DPM disabled

As MGPU fan boost feature will be definitely not needed when
DPM is disabled. So, there is no need to error out.

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>