drm/v3d: Invalidate the caches from the outside in.

This would be a fairly obscure race, but let's make sure we don't ever
lose it.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-6-eric@anholt.net


Reviewed-by: Dave Emett <david.emett@broadcom.com>