drm/i915: fix gen2-gen3 backlight set

Citing Jani's response to Imre's question in the review discussion:

> According to the gen2/3 bspec I have, the correct mask is
> BACKLIGHT_DUTY_CYCLE_MASK_PNV only in case of IS_PINEVIEW(dev), for
> everything else it's BACKLIGHT_DUTY_CYCLE_MASK.

What you say is correct, but we've treated all gen2/3 similar to PNV
since

commit ca88479c1c3b7b1a9f94320745f5331e1de77f80
Author: Keith Packard <keithp@keithp.com>
Date:   Fri Nov 18 11:09:24 2011 -0800

    drm/i915: Treat pre-gen4 backlight duty cycle value consistently

i.e. we only use the high 15 bits for all gen2/3. For non-PNV this just
means the lowest bit is always zero. For PNV the lowest bit has a
different meaning in both the PWM freq and duty cycle fields.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Make the commit message less empty.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -555,7 +555,7 @@ static void i9xx_set_backlight(struct intel_connector *connector, u32 level)
 {
 	struct drm_device *dev = connector->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	u32 tmp;
+	u32 tmp, mask;
 
 	if (is_backlight_combination_mode(dev)) {
 		u32 max = intel_panel_get_max_backlight(connector);
@@ -570,10 +570,14 @@ static void i9xx_set_backlight(struct intel_connector *connector, u32 level)
 		pci_write_config_byte(dev->pdev, PCI_LBPC, lbpc);
 	}
 
-	if (INTEL_INFO(dev)->gen < 4)
+	if (IS_GEN4(dev)) {
+		mask = BACKLIGHT_DUTY_CYCLE_MASK;
+	} else {
 		level <<= 1;
+		mask = BACKLIGHT_DUTY_CYCLE_MASK_PNV;
+	}
 
-	tmp = I915_READ(BLC_PWM_CTL) & ~BACKLIGHT_DUTY_CYCLE_MASK;
+	tmp = I915_READ(BLC_PWM_CTL) & ~mask;
 	I915_WRITE(BLC_PWM_CTL, tmp | level);
 }