radeonsi: enable zerovram for Rocket League

Fixes corruption on game startup.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1888



Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
(cherry picked from commit 5afbe87d)