graphics/drm-stable-kmod, graphics/drm-next-kmod: Update to latest snapshot

This fixes a regression on CURRENT after some changes to lkpi in base r334482

Approved by: jmd (maintainer, implicit, regression fix)
MFH: 2018Q2
X-MFH-note: include r471110, better support for old GPUs in stable-kmod
Tested by: Johannes Lundberg