anv: Emit a dummy MEDIA_VFE_STATE before switching from GPGPU to 3D

There is an object-level  preemption workaround which requires this.
However, even without object-level preemption, we seem to have issues
with geometry flickering when 3D and compute are combined in the same
batch and this appears to fix it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109630
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111267


Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit bc612536)