graphics/drm-legacy-kmod: Update snapshot

Update graphics/drm-legacy-kmod to the latest snapshot. This includes
FreeBSD r342182 by markj, which removes support code for UMS in radeonkms.
This code was never used, but contains an integrer overflow bug.

Original FreeBSD base commit:
https://svnweb.freebsd.org/changeset/base/342182

Approved by: jmd (maintainer, implicit)
MFH: 2018Q4