drm/i915: make rc6 module parameter read-only

People have been getting confused and thinking this is a runtime control.

Cc: stable@vger.kernel.org
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -64,7 +64,7 @@ MODULE_PARM_DESC(semaphores,
 		"Use semaphores for inter-ring sync (default: -1 (use per-chip defaults))");
 
 int i915_enable_rc6 __read_mostly = -1;
-module_param_named(i915_enable_rc6, i915_enable_rc6, int, 0600);
+module_param_named(i915_enable_rc6, i915_enable_rc6, int, 0400);
 MODULE_PARM_DESC(i915_enable_rc6,
 		"Enable power-saving render C-state 6. "
 		"Different stages can be selected via bitmask values "