[d3d12] fix possible crash on GPUs that only support SM5.1

[teoxoy]

Connections
Fixes Bug 2011999.
Regression from #8576.

Description
Adapt code to expect SM5.1.

Testing
-

Squash or Rebase?
n/a

[ErichDonGubler]

I'm confused about this. Is CheckFeatureSupport actually returning 5.1 without an error? I'm guessing that's buggy behavior from drivers that were written at the time?

[teoxoy]

It's intentional:

When used with the ID3D12Device::CheckFeatureSupport function, before calling the function initialize the HighestShaderModel field to the highest shader model that your application understands. After the function completes successfully, the HighestShaderModel field contains the highest shader model that is both supported by the device and no higher than the shader model passed in.

from https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_feature_data_shader_model#remarks

[jimblandy]

This looks good to me. D3D_SHADER_MODEL_5_1 is the oldest value in the D3D_SHADER_MODEL enumeration.