drm/amdgpu: add read amdgpu_gfxoff status in debugfs

 Add interface for SMU12 device, used by UMR.

v2: fix code style

Signed-off-by: Jinzhou.Su <Jinzhou.Su@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>