allocator: remove backend parameter in allocator_autocreate_with_drm_fd

This allows for creating an allocator without referencing a backend. This is useful for things like !4055 which want to create renderer/allocator resources without using the DRM backends.

Since we only use the backend capabilities here we can simply pass them in directly. This allows other locations to create an allocator even if they don't have a backend. They can simply specify the caps they want instead.