drm/radeon: re-enable sw ACR support on pre-DCE4

HW ACR support may have issues on some older chips, so
use SW ACR for now until we've tested further.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: Rafał Miłecki <zajec5@gmail.com>

drivers/gpu/drm/radeon/r600_hdmi.c

@@ -451,6 +451,7 @@ void r600_hdmi_setmode(struct drm_encoder *encoder, struct drm_display_mode *mod
 	}
 
 	WREG32(HDMI0_ACR_PACKET_CONTROL + offset,
+	       HDMI0_ACR_SOURCE | /* select SW CTS value - XXX verify that hw CTS works on all families */
 	       HDMI0_ACR_AUTO_SEND); /* allow hw to sent ACR packets when required */
 
 	WREG32(HDMI0_VBI_PACKET_CONTROL + offset,