Add updating entry for DRM

Update messaging for which drm module to install. Add guidance on what
hardware is supported (which should be copied into the release
notes). Note: the in tree drivers are abandonware. There has been no
organized support for them for many years, and the plan is to still
remove them for all but arm once the transition to drm-*kmod is
complete. Also note that WITHOUT_MODULE_DRM and WITHOUT_MODULE_DRM2
should generally be added to src.conf for anybody using the drm-*kmod
ports. That will become default in 13 soon, however.

Approved by: FreeBSD Graphics Team
Relnotes: Yes
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D17451