drm/amdgpu: Cache result of last reset at reset domain level.

Will be read by executors of async reset like debugfs.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>