[dx12] Get test write_texture_subset to pass with dx12

[nical]

Description

In #3063 a test was added which appears to fail on dx12. The test creates a texture and uploads to a subset of the texture using queue.write_texture.

Repro steps

in wgpu/tests/write_texture.rs, replace

let parameters = TestParameters::default().backend_failure(wgpu::Backends::DX12);

with

let parameters = TestParameters::default();

and run the test on Windows suing the dx12 backend.

Platform

Windows