drm/amd/powerplay: enable clock stretch feature for polaris

Power saving feature which reduces the amount of
voltage needed for specific engine clocks.

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