KVM: arm64: Use mtree_empty() to determine if SMCCC filter configured

The smccc_filter maple tree is only populated if userspace attempted to
configure it. Use the state of the maple tree to determine if the filter
has been configured, eliminating the VM flag.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231004234947.207507-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>