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: 17e4660a (drm/etnaviv: implement per-process address spaces on MMUv2)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>