radeonsi: avoid sending GS_EMIT in shaders without outputs

Fixes GPU hangs.

Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107857


Signed-off-by: Józef Kucia <joseph.kucia@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit e24a4e05)