drm/amdgpu: expose amdgpu_bo_create_shadow()

Exposed amdgpu_bo_create_shadow() will be needed
for amdgpu_vm handling.

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>