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.

[PatTheMav]

While I think that it makes sense to share generic D3D code between a DirectX11 and DirectX12 renderer, I don't see much value in splitting those off without the new renderer in place.

The change should be coupled with a PR for the new renderer and the shared functionality should probably be placed in the shared directory as either a header-only or object library (so that both renderers can pull the functionality from the same place).

Until that new renderer is ready for production and reviewable, I don't see much benefit in merging it before that.

[wanhongqing123]

reopen when #12907 is merged.