KVM: arm64: vgic-v3: Consolidate userspace access for MMIO registers

For userspace accesses to GICv3 MMIO registers (and related data),
vgic_v3_{get,set}_attr are littered with {get,put}_user() calls,
making it hard to audit and reason about.

Consolidate all userspace accesses in vgic_v3_attr_regs_access(),
making the code far simpler to audit.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>