virtgpu: Add vdrm helper for drivers that support virtgpu native context
Since both AMD and intel native context implementations ended up looking pretty similar to the msm implementation, I refactored out some common bits and made a helper.
The next step is to add a vtest implementation of the helper for easier testing. But that will require vtest support for fence fd's which will end up depending on virgl/virglrenderer!1138 (merged). (And it will also require a vtest private interface for syncobj support.. I've not quite decided how that should look yet.)
@Hazematman I enlisted a subset of one of your patches from !21570