drm/edid: fix reduced blanking support check

The reduced blanking bit is valid only for CVT, indicated by display
range limits flags 0x04.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5dea5ee24065450716bbc177dd6850d3193dbeec.1648477901.git.jani.nikula@intel.com

drivers/gpu/drm/drm_edid.c

@@ -2408,7 +2408,7 @@ is_rb(struct detailed_timing *t, void *data)
 	if (!is_display_descriptor(r, EDID_DETAIL_MONITOR_RANGE))
 		return;
 
-	if (r[15] & 0x10)
+	if (r[10] == DRM_EDID_CVT_SUPPORT_FLAG && r[15] & 0x10)
 		*(bool *)data = true;
 }