brlock: Always pass argument to br_read_lock/unlock

The kvm argument is not passed to br_read_lock/unlock, this works for
the barrier implementation because the argument is not used. This ever
breaks if another lock implementation is used.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>