d3d12: only clear D3D12_DIRTY_INDEX_BUFFER when valid

If the first draw to a batch is non-indexed, we shouldn't clear D3D12_DIRTY_INDEX_BUFFER. This will lead to the next draw missing state-updates, which in turn leads to incorrect rendering.

We only need this conditional treatment for the index-buffer state, because it's the only state that is conditionally validated. This code could probably need some cleanup to make this stuff more obvious, but let's leave that for another day.

This fixes rendering-glitches in Variform by Kewlers, and probably also in Blender 2.83.