drm: add missing header guards to drm_internal.h

Including the file twice leads to errors.

Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/6e7[e-mail]

drivers/gpu/drm/drm_internal.h

···
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#ifndef __DRM_INTERNAL_H__
+#define __DRM_INTERNAL_H__
+
 #include <linux/kthread.h>
 #include <linux/types.h>
 
···
 /* 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__ */