drm/i915: Use intel_panel_actually_set_backlight() to disable PWM backlight

Use intel_panel_actually_set_backlight() instead of a direct
call to pwm_config() in pwm_disable_backlight().

The main benefit is consistent debug logging when we turn off the
backlight. Currently we see nothing in dmesg which made me wonder
whether the backlight was even getting turned off properly.

The second benefit is consistency; This is what we do for all
the other backlight implementations.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024155208.18987-1-ville.syrjala@linux.intel.comReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Acked-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -763,7 +763,7 @@ static void pwm_disable_backlight(const struct drm_connector_state *old_conn_sta
 	struct intel_panel *panel = &connector->panel;
 
 	/* Disable the backlight */
-	pwm_config(panel->backlight.pwm, 0, CRC_PMIC_PWM_PERIOD_NS);
+	intel_panel_actually_set_backlight(old_conn_state, 0);
 	usleep_range(2000, 3000);
 	pwm_disable(panel->backlight.pwm);
 }