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/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 9e02dd7..3999afa 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -2238,11 +2238,6 @@
 		_intel_edp_backlight_off(intel_dp);
 }
 
-static const char *state_string(bool enabled)
-{
-	return enabled ? "on" : "off";
-}
-
 static void assert_dp_port(struct intel_dp *intel_dp, bool state)
 {
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
@@ -2252,7 +2247,7 @@
 	I915_STATE_WARN(cur_state != state,
 			"DP port %c state assertion failure (expected %s, current %s)\n",
 			port_name(dig_port->port),
-			state_string(state), state_string(cur_state));
+			onoff(state), onoff(cur_state));
 }
 #define assert_dp_port_disabled(d) assert_dp_port((d), false)
 
@@ -2262,7 +2257,7 @@
 
 	I915_STATE_WARN(cur_state != state,
 			"eDP PLL state assertion failure (expected %s, current %s)\n",
-			state_string(state), state_string(cur_state));
+			onoff(state), onoff(cur_state));
 }
 #define assert_edp_pll_enabled(d) assert_edp_pll((d), true)
 #define assert_edp_pll_disabled(d) assert_edp_pll((d), false)