backend/headless: stop picking a DRM FD

Sometimes the headless backend is used standalone with the Pixman
renderer, sometimes it's used together with another backend which
has already picked a DRM FD. In both of these cases it doesn't make
sense to pick a DRM FD.

Broadly speaking the headless backend doesn't really care which DRM
device is used for the buffers it receives. So it doesn't really
make sense to tie it to a particular DRM device.

Let the backend users (e.g. wlr_renderer_autocreate) open an arbitrary
DRM FD as needed instead.

include/backend/headless.h

@@ -8,7 +8,6 @@
 
 struct wlr_headless_backend {
 	struct wlr_backend backend;
-	int drm_fd;
 	struct wl_display *display;
 	struct wl_list outputs;
 	size_t last_output_num;