KVM: ARM: Reuse kvm_call_hyp in vcpu_init_hyp_mode

Instead of directly and manually callin the hypercall into the KVM init
code, use the kvm_call_hyp function, which only requires a small abuse
of the prototype in exchange for much nicer C code.

Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>