drm/amd/powerplay: add function get_fan_speed_percent for navi10

add callback function get_fan_speed_percent for navi10 asic

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>