drm/amdgpu/df: implement df v3_6 callback functions (v2)

New df helpers for 3.6.

v2: switch to using df 3.6 headers (Alex)

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>