drm/i915: Correct typo for __i915_gem_active_get_rcu in a comment

I mistyped and added an extra _request_ to __i915_gem_active_get_rcu()
Also, the same happened to another comment for i915_gem_active_get_rcu()

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470758602-1338-1-git-send-email-chris@chris-wilson.co.uk