drm/i915: use backlight legacy combination mode also for i915gm/i945gm

i915gm and i945gm also seem to use and need the legacy combination mode
bit in BLC_PWM_CTL.

v2: Also do this for i915gm (Ville).

Reported-and-tested-by: Luis Ortega <luiorpe1@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75001
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -698,7 +698,7 @@ static void i9xx_enable_backlight(struct intel_connector *connector)
 		freq /= 0xff;
 
 	ctl = freq << 17;
-	if (IS_GEN2(dev) && panel->backlight.combination_mode)
+	if (panel->backlight.combination_mode)
 		ctl |= BLM_LEGACY_MODE;
 	if (IS_PINEVIEW(dev) && panel->backlight.active_low_pwm)
 		ctl |= BLM_POLARITY_PNV;
@@ -979,7 +979,7 @@ static int i9xx_setup_backlight(struct intel_connector *connector)
 
 	ctl = I915_READ(BLC_PWM_CTL);
 
-	if (IS_GEN2(dev))
+	if (IS_GEN2(dev) || IS_I915GM(dev) || IS_I945GM(dev))
 		panel->backlight.combination_mode = ctl & BLM_LEGACY_MODE;
 
 	if (IS_PINEVIEW(dev))