drm/amd/powerplay: enable gpu_busy_percent sys interface for renoir (v2)

To get the value of gpu_busy_percent, it needs to realize
get_current_activity_percent and get_metrics_table.
The framework of renoir smu is different from old ones like raven. It
needs to realize get_current_activity_percent and get_metrics_table in
renoir_ppt.c like navi10.

v2: remove unused variable (Alex)

Signed-off-by: changzhu <Changfeng.Zhu@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>