drm/amdgpu: validate duplicates first

Most VM BOs end up in the duplicates list, validate it
first make -ENOMEM less likely.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <David1.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>