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.h b/drivers/gpu/drm/i915/i915_drv.h
index 104bd18..eb7bb97 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -102,6 +102,11 @@
 	return v ? "yes" : "no";
 }
 
+static inline const char *onoff(bool v)
+{
+	return v ? "on" : "off";
+}
+
 enum pipe {
 	INVALID_PIPE = -1,
 	PIPE_A = 0,