drm2: Update deprecation message

Update the deprecation message in the drm2 (aka legacy drm) drivers to point
towards the graphics/drm-kmod ports for all architectures, not just amd64.
drm-kmod has support for more architectures these days, and the
graphics/drm-legacy-kmod port is being deprecated.

Approved by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26174