[dx12] Fill out max_framebuffer_layers and max_per_stage_resources

[kvark]

Helps with https://github.com/gfx-rs/portability/issues/242#issuecomment-771881593 PR checklist:

• [ ] make succeeds (on *nix)
• [ ] make reftests succeeds
• [ ] tested examples with the following backends:

[kvark]

bors r+

[bors[bot]]

Build failed:

• Windows Nightly

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable