drm/i915: Remove orphaned prototype gen6_set_pm_mask()

The function was removed in:

  commit 037bde19
  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>