drm/amdgpu: create shadow bo using amdgpu_bo_create_shadow()

Shadow BOs are only needed for vm code so call amdgpu_bo_create_shadow()
directly instead of depending on amdgpu_bo_create().

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>