drm2: Further improve deprecation message

Further improve the drm2 deprecation message, only displaying information
about the port for relevant architectures, and skipping the message
completely from arm, which uses some parts of drm2 still.

This is mostly intended to be merged to 12, since the base bits of drm2 on
FreeBSD 13 are only really used on arm.

Reviewed by: manu, mmel
Approved by: manu
MFC after: 3 days
X-MFC-with: r364737
Differential Revision: https://reviews.freebsd.org/D26275