KVM: arm64: vgic-its: Make vgic_its_cmd_handle_mapi similar to other handlers

vgic_its_cmd_handle_mapi has an extra "subcmd" argument, which is
already contained in the command buffer that all command handlers
obtain from the command queue. Let's drop it, as it is not that
useful.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>