drm/kms probe speedups
Systemd waits for oneshot units like plymouth-switch-root.service to finish and this can block the boot. Specifically plymouth-switch-root.service must complete before systemd in the initrd will exec the systemd from the real rootfs. This means that systemd inside the initrd waits for the
ExecStart=-/usr/bin/plymouth update-root-fs --new-root-dir=/sysroot command to complete.
When the actual rootfs has just been mounted on /sysroot, then systemd will start plymouth-switch-root.service. If this happens while we are still probing drm/kms outputs then this probing will delay the switching to the actual rootfs. On my mail workstation I've seen delays as high as 2.4 seconds caused by this.
The commits in this merge-req together reduce the delay to around 0.5 seconds which is much better.