drm/i915: add DOC: headline to RC6 kernel-doc

Without the DOC:, kernel-doc confuses the documentation block for
something else.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453101588-18008-3-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -32,6 +32,8 @@
 #include <linux/module.h>
 
 /**
+ * DOC: RC6
+ *
  * RC6 is a special power stage which allows the GPU to enter an very
  * low-voltage mode when idle, using down to 0V while at this stage.  This
  * stage is entered automatically when the GPU is idle when RC6 support is