drm/amdgpu: add amdgpu_bo_vm bo type

Add new BO subclass that will be used by amdgpu vm code.

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>