drm: Update deprecation message

Update the deprecation message in the drm1 (aka legacy drm or drm-legacy)
drivers to not point towards the drm-legacy-kmod port, as that is being
deprecated.

This is a direct commit to stable/12 since the drm1 code has been removed
from 13 already.

Reviewed by: imp
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D26175