drm/amd/pm: use attr_update if the attr has it

use attr_update if its available as part of the attribute.
default_attr_update was used even if attr->attr_update is true.

Signed-off-by: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: Shashank Sharma <Shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>