Neuter server-side wl_drm

wl_drm has been replaced with wp_linux_dmabuf for buffer creation purposes back in 2015, so let’s remove server-side support for it.

This series stops advertising any format on this interface, removes support for creating a wl_buffer from wl_drm with an invalid_format error (since no format is advertised anymore), and stubs or removes code expecting to be able to use it.

The client-side usage of wl_drm isn’t changed in any way, this is left for a future MR.