drm/amd/powerplay: simplify the code of [get|set]_activity_monitor_coeff

use smu_update_table_with_arg to replace old code logic

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>