msm-fb-refresher: use initfs hook (#1438)

The `msm-fb-refresher` updates the screen for msm based devices. It is
not needed for all devices, so we had some extra code in the initramfs,
that would only add it when the `deviceinfo_msm_refresher` variable was
set. However, we are able now to add files to initramfs hooks, so this
hack can be removed and simplify everything.

Changes:
* Remove `deviceinfo_msm_refresher` from all deviceinfos
* Add sanity check for it
* Move all `deviceinfo` sanity checks to an extra function
* `postmarketos-mkinitfs`: remove code for msm refresher
* `msm-fb-refresher`: add initramfs hook