staging: etnaviv: remove presumption of BO addresses

We never pass the GPU addresses of BOs to userspace, so userspace can
never specify the correct address.  Hence, this code serves no useful
purpose, and can be removed.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>