render: replace wlr_renderer_get_drm_fd() with drm_dev_id

wlr_renderer_get_drm_fd() forces the renderer to open the DRM node
and keep the FD around for no good reason. Instead, use dev_t to
refer to a DRM device.

This also removes footguns related to FD ownership.

include/render/gles2.h

@@ -41,8 +41,8 @@ struct wlr_gles2_tex_shader {
 struct wlr_gles2_renderer {
 	struct wlr_renderer wlr_renderer;
 
+	dev_t drm_dev_id;
 	struct wlr_egl *egl;
-	int drm_fd;
 
 	struct wlr_drm_format_set shm_texture_formats;