Revert "drm/vgem: fix cache synchronization on arm/arm64"

commit 7e9e5ead ("drm/vgem: fix cache synchronization on arm/arm64")
broke all of the !llc i915-vgem coherency tests in CI, and left the HW
very, very unhappy (which is even more scary).

Fixes: 7e9e5ead ("drm/vgem: fix cache synchronization on arm/arm64")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rob Clark <robdclark@chromium.org>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190801124458.24949-1-chris@chris-wilson.co.uk