drm/etnaviv: only check if the cmdbuf is inside the linear window on MMUv1

There is no linear window on MMUv2 and the FE can access the full 4GB
address space either directly (as long as the MMU isn't configured) or
through the MMU, once it is up.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>