drm: add missing header guards to drm_crtc_internal.h

Including the file twice can lead to errors.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7ad51d9fb9c42c9901c5c1d5d16e32e857da9758.1709898638.git.jani.nikula@intel.com

drivers/gpu/drm/drm_crtc_internal.h

@@ -32,6 +32,9 @@
  * and are not exported to drivers.
  */
 
+#ifndef __DRM_CRTC_INTERNAL_H__
+#define __DRM_CRTC_INTERNAL_H__
+
 #include <linux/err.h>
 #include <linux/types.h>
 
@@ -305,3 +308,5 @@ drm_edid_load_firmware(struct drm_connector *connector)
 	return ERR_PTR(-ENOENT);
 }
 #endif
+
+#endif /* __DRM_CRTC_INTERNAL_H__ */