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.c

@@ -2003,6 +2003,7 @@ drm_head_log_info(struct drm_head *head, const char *msg)
  *
  * @param head The head to update.
  * @param conn The DRM connector object.
+ * @param drm_device udev device pointer
  * @returns 0 on success, -1 on failure.
  *
  * Updates monitor information and connection status. This may schedule a