ecore_drm2: refcount fbs

ecore_drm2: refcount fbs

Removes the previous "busy" flag, as now we might have an fb attached to
multiple outputs at once, and need to be careful to destroy them only
after they've been removed from all outputs.

Removed the old "busy_set" API which nothing used, and renames fb_destroy
to fb_discard to make it more clear that it's not immediately destroyed.

It's all beta api, so I can do this.