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

commit b852c985010a77c850b7548d64bbb964ca462b02 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>