drm/i915: Remove orphaned prototype gen6_set_pm_mask()

The function was removed in:

  commit 037bde19a43e299d30f0490bba9be32ab355975c
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Thu Mar 27 08:24:19 2014 +0000

      Revert "drm/i915: Disable/Enable PM Intrrupts based on the current freq."

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -2298,8 +2298,6 @@ __printf(3, 4)
 void i915_handle_error(struct drm_device *dev, bool wedged,
 		       const char *fmt, ...);
 
-void gen6_set_pm_mask(struct drm_i915_private *dev_priv, u32 pm_iir,
-							int new_delay);
 extern void intel_irq_init(struct drm_i915_private *dev_priv);
 extern void intel_hpd_init(struct drm_i915_private *dev_priv);
 int intel_irq_install(struct drm_i915_private *dev_priv);