drm/amdgpu: introduce struct amdgpu_bo_user

Implement a new struct amdgpu_bo_user as subclass of
struct amdgpu_bo and a function to created amdgpu_bo_user
bo with a flag to identify the owner.

v2: amdgpu_bo_to_amdgpu_bo_user -> to_amdgpu_bo_user()

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>