grpahics/drm-current-kmod: Update snapshot

Update graphics/drm-current-kmod to the latest snapshot. This breaks out
the ttm layer to its own module to have things initialized in the correct
order.

Fixes FreeBSDDesktop drm-kms issue #137

Submitted by: johalun
Approved by: jmd (maintainer, implicit)
MFH: 2019Q1 (implicit approval, drm driver blanket)