Simple, create this hook in /etc/pacman.d/hooks/nvidia.hook, so that mkinitcpio will be triggered each time there is a nvidia-dkms update. You can replace Target=nvidia-dkms with Targer=nvidia if needed.
[Trigger]
Operation=Install
Operation=Upgrade
Operation=Remove
Type=Package
Target=nvidia-dkms
[Action]
Depends=mkinitcpio
When=PostTransaction
Exec=/bin/sh -c ‘/usr/bin/mkinitcpio -P’