drm: document drm_auth.c

Also extract drm_auth.h for nicer grouping.

v2: Nuke the other comments since they don't really explain a lot, and
within the drm core we generally only document functions exported to
drivers: The main audience for these docs are driver writers.

v3: Limit the exposure of drm_master internals by only including
drm_auth.h where it is neede (Chris).

v4: Spelling polish (Emil).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_ioctl.c

@@ -30,6 +30,7 @@
 
 #include <drm/drmP.h>
 #include <drm/drm_core.h>
+#include <drm/drm_auth.h>
 #include "drm_legacy.h"
 #include "drm_internal.h"
 #include "drm_crtc_internal.h"