Reuse drmModeGetResources() call in drm_backend_create()

The following discussion from !441 (merged) should be addressed:

• @pq started a discussion: (+2 comments)

  This is a TODO suggestion for later:

  I see there are several functions called from drm_backend_create() that work like this:

  • drmModeGetResources()
  • do stuff
  • free resources

  It would be better if resources were passed as an argument into these functions, so that we only need to call drmModeGetResources() once during drm_backend_create() (or another new function that is responsible for the resources init).

  Feel free to convert this discussion into a new issue.