drm/amd/powerplay: change dmesg log level in powerplay

Use pr_debug to prevent spamming unimportant dmesg.

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