drm/amdgpu: add some pptable funcs for cyan skilfish(v3)

Add print_clk_levels and read_sensor pptable funcs for
cyan skilfish.

v2: keep consitency and add get_gpu_metrics callback.
v3: use sysfs_emit_at() in sysfs show function.

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>