drm/i915/dpcd_bl: Unbreak enable_dpcd_backlight modparam

Looks like I accidentally made it so you couldn't force DPCD backlight
support on, whoops. Fix that.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 17f5d579 ("drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel")
Cc: Adam Jackson <ajax@redhat.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200413214407.1851002-1-lyude@redhat.com

drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c

@@ -358,6 +358,7 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 	 */
 	if (i915->vbt.backlight.type !=
 	    INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	    i915_modparams.enable_dpcd_backlight != 1 &&
 	    !drm_dp_has_quirk(&intel_dp->desc, intel_dp->edid_quirks,
 			      DP_QUIRK_FORCE_DPCD_BACKLIGHT)) {
 		drm_info(&i915->drm,