drm/i915: add onoff utility function

Add a common function to return "on" or "off" string based on the
argument, and drop the local versions of it.

This is the onoff version of

commit 42a8ca4cb4a48ddbf40e8edb291425e76bcdc230
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Aug 27 16:23:30 2015 +0300

    drm/i915: add yesno utility function

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452768814-29787-1-git-send-email-jani.nikula@intel.com
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index f17a2b0..975af35 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -1335,8 +1335,8 @@
 		return 0;
 
 	DRM_DEBUG_KMS("waiting for GT wells to go %s (%08x)\n",
-			wait_for_on ? "on" : "off",
-			I915_READ(VLV_GTLC_PW_STATUS));
+		      onoff(wait_for_on),
+		      I915_READ(VLV_GTLC_PW_STATUS));
 
 	/*
 	 * RC6 transitioning can be delayed up to 2 msec (see
@@ -1345,7 +1345,7 @@
 	err = wait_for(COND, 3);
 	if (err)
 		DRM_ERROR("timeout waiting for GT wells to go %s\n",
-			  wait_for_on ? "on" : "off");
+			  onoff(wait_for_on));
 
 	return err;
 #undef COND