MFC: r364737, r365264 and r365287

Together, these three revisions improve the drm2 (aka legacy drm or
drm-legacy) drivers to point towards graphics/drm-kmod where relevant, and
to remove references to graphics/drm-legacy-kmd as that is being deprecated.
Since part of the drm2 drivers are still used on arm, arm is currently
excluded from the deprecation message.

Approved by: imp, manu (implicit, MFC)