KVM: arm/arm64: vgic-its: Properly check the unmapped coll in DISCARD handler

Discard is supposed to fail if the collection is not mapped to any
target redistributor. We currently check if the collection is mapped
by "ite->collection" but this is incomplete (e.g., mapping a LPI to
an unmapped collection also results in a non NULL ite->collection).
What actually needs to be checked is its_is_collection_mapped(), let's
turn to it.

Also take this chance to remove an extra blank line.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20200114112212.1411-1-yuzenghui@huawei.com