drm/radeon/si: add some missing regs to the VM reg checker

commit f418b88aad0c42b4caf4d79a0cf8d14a5d0a2284 upstream.

This register is needed for streamout to work properly.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>