When the caller wants to perform a modeset, they will need to pass
DRM_MODE_ATOMIC_ALLOW_MODESET in the atomic commit flags, otherwise the driver
will fail the commit. libliftoff also needs to provide these flags in its
test-only commits.
From the kernel docs:
> -EINTR, -EAGAIN or -ERESTARTSYS, if the IOCTL should be restarted. This can
> either be due to a pending signal, or because the driver needs to completely
> bail out to recover from an exceptional situation like a GPU hang. From a
> userspace point of view all errors are treated equally.
We're not handling ERESTARTSYS because it's non-POSIX.
