drm-backend: fix deinit_planes

Commit "drm-backend: move code to init/deinit planes to specific functions" lost a chunk of drm_output_deinit() when moving code into drm_output_deinit_planes(). Reinstate the missing chunk.

This fixes an endless loop over weston_compositor::plane_list when you start with three monitors connected, unplug and re-plug one.

Fixes: 3be23eff

Cc @daniels @leandrohrb

I missed this in my review of the faulty commit.