drm/i915: set backlight duty cycle after backlight enable for gen4

For reasons I can't claim to fully understand gen4 seems to require
backlight duty cycle setting after the backlight has been enabled, or
else black screen follows. I don't have documentation for the correct
sequence on gen4 either. Confirmed on Dell Latitude D630 and MacBook4,1.

This fixes a regression introduced by
commit b35684b8fa94e04f55fd38bf672b737741d2f9e2
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Nov 14 12:13:41 2013 +0200

    drm/i915: do full backlight setup at enable time

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=75791
Reported-and-tested-by: mcy@lm7.fr
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79423
Reported-and-tested-by: Marc Milgram <mmilgram@redhat.com>
Cc: Stable <stable@vger.kernel.org> # 3.14+
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -798,9 +798,6 @@ static void i965_enable_backlight(struct intel_connector *connector)
 	ctl = freq << 16;
 	I915_WRITE(BLC_PWM_CTL, ctl);
 
-	/* XXX: combine this into above write? */
-	intel_panel_actually_set_backlight(connector, panel->backlight.level);
-
 	ctl2 = BLM_PIPE(pipe);
 	if (panel->backlight.combination_mode)
 		ctl2 |= BLM_COMBINATION_MODE;
@@ -809,6 +806,8 @@ static void i965_enable_backlight(struct intel_connector *connector)
 	I915_WRITE(BLC_PWM_CTL2, ctl2);
 	POSTING_READ(BLC_PWM_CTL2);
 	I915_WRITE(BLC_PWM_CTL2, ctl2 | BLM_PWM_ENABLE);
+
+	intel_panel_actually_set_backlight(connector, panel->backlight.level);
 }
 
 static void vlv_enable_backlight(struct intel_connector *connector)