radeonsi: ensure that VGT_GS_MODE is sent when necessary

radeonsi: ensure that VGT_GS_MODE is sent when necessary

Specifically, when the API switches from using a GS to not using a GS and then
back to using the same GS again, we do not have to re-send all the GS state,
but we do have to send VGT_GS_MODE. So make VGT_GS_MODE consistently be a part
of the VS state.

This fixes a rendering bug in Dolphin, but surely other applications are
affected as well.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93648
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>