radeonsi/gfx9: fix a hang with an empty first IB

This packet causes the no-op IB detection to fail, so the IB is always
submitted. Also fix the no-op IB detection by moving the begin call.

Cc: 18.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>