backend-drm: Get optional WESTON_TRANSFORM from udev.

Should allow configuration of rotation along the lines of:

ACTION!="remove", SUBSYSTEM=="drm", SUBSYSTEMS=="pci|usb|platform", KERNEL=="card*", ENV{WL_OUTPUT}="VGA-1", ENV{WESTON_TRANSFORM}="rotate-270"

libweston/backend-drm/drm-internal.h

@@ -614,7 +614,7 @@ struct drm_mode *
 drm_output_choose_mode(struct drm_output *output,
 		       struct weston_mode *target_mode);
 void
-update_head_from_connector(struct drm_head *head);
+update_head_from_connector(struct drm_head *head, struct udev_device *drm_device);
 
 void
 drm_mode_list_destroy(struct drm_backend *backend, struct wl_list *mode_list);