avmodel: avoid write-locking the mutex when deleting the renderer

During a resize event the the addRenderer function will regenerate a Renderer of the new size. The Renderer destructor will eventually trigger a write-lock in VideoProvider which may deadlock with frame updates to other renderers.

Gitlab: #1161
Change-Id: I3f3b99f6f78ac94218213130d42efcf0884b38fa