drm/etnaviv: reinstate MMUv1 command buffer window check

drm/etnaviv: reinstate MMUv1 command buffer window check

The switch to per-process address spaces erroneously dropped the check
which validated that the command buffer is mapped through the linear
apperture as required by the hardware. This turned a system
misconfiguration with a helpful error message into a very hard to
debug issue. Reinstate the check at the appropriate location.

Fixes: 204f23cd3e78 (drm/etnaviv: implement per-process address spaces on MMUv2)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>