drm: add missing header guards to drm_internal.h

Including the file twice leads to errors.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6e744360513e581765147ea7b1e693f4bffe03a9.1709749576.git.jani.nikula@intel.com

drivers/gpu/drm/drm_internal.h

@@ -21,6 +21,9 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#ifndef __DRM_INTERNAL_H__
+#define __DRM_INTERNAL_H__
+
 #include <linux/kthread.h>
 #include <linux/types.h>
 
@@ -276,3 +279,5 @@ void drm_framebuffer_debugfs_init(struct drm_device *dev);
 /* drm_edid.c */
 void drm_edid_cta_sad_get(const struct cea_sad *cta_sad, u8 *sad);
 void drm_edid_cta_sad_set(struct cea_sad *cta_sad, const u8 *sad);
+
+#endif /* __DRM_INTERNAL_H__ */