drm/amdgpu/dpm: don't add pwm attributes if DPM is disabled

PWM fan control is only available with DPM.  There is no non-DPM
support on amdgpu, so we should never get a crash here because
the sysfs nodes would never be created in the first place. Add the
check just in case to be on the safe side.

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