drm/i915: spt does not have pch backlight override bit

SPT is always in the PCH override mode, and the bit MBZ. Only set
override on LPT.

v2: check for PCH version (Ville)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -775,7 +775,8 @@ static void bdw_enable_backlight(struct intel_connector *connector)
 	if (panel->backlight.active_low_pwm)
 		pch_ctl1 |= BLM_PCH_POLARITY;
 
-	/* BDW always uses the pch pwm controls. */
+	/* After LPT, override is the default. */
+	if (HAS_PCH_LPT(dev_priv))
 		pch_ctl1 |= BLM_PCH_OVERRIDE_ENABLE;
 
 	I915_WRITE(BLC_PWM_PCH_CTL1, pch_ctl1);