staging: etnaviv: mmuv1: ensure we unmap all entries

Ensure that we unmap all MMU entries when unmapping a region.  We
fail to do this because we assume that the return value from the
unmap method should be zero.  It should be the size of entry which
has been unmapped.

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