drm/i915: Remove incorrect comment about struct mutex

This statenment became false here:

commit 4fc688ce79772496503d22263d61b071a8fb596e
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Nov 2 11:14:01 2012 -0700

    drm/i915: protect RPS/RC6 related accesses (including PCU) with a new mutex

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -904,8 +904,6 @@ struct intel_gen6_power_mgmt {
 	struct work_struct work;
 	u32 pm_iir;
 
-	/* The below variables an all the rps hw state are protected by
-	 * dev->struct mutext. */
 	u8 cur_delay;
 	u8 min_delay;
 	u8 max_delay;