libobs-d3d11: Put d3d11 and d3d12 common functions together

[wanhongqing123]

Description

Put d3d11 and d3d12 common functions together.

Motivation and Context

I am currently writing the implementation of D3D12. During this process, I found that there are many functions that are needed by both D3D11 and D3D12, so I summarized these functions together.

How Has This Been Tested?

I have tested in my compute.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.