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

PWM fan control is only available with DPM.  If DPM disabled,
don't expose the PWM fan controls to avoid a crash.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=92524



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org