drm: Update to Linux 4.15.18

* Create /dev/dri/renderD* devices
Generally handle /dev entries creation as close to Linux as possible.

* Add drm master support
Sync authentification code with Li

drm: Update to Linux 4.15.18

* Create /dev/dri/renderD* devices
Generally handle /dev entries creation as close to Linux as possible.

* Add drm master support
Sync authentification code with Linux 4.15.18

* handle vm_mm->mmap_sem in ttm page fault operations

* Update dma-fence code from OpenBSD

* This commit contains sleep/wakeup and other changes
from Matthew Dillon <dillon@apollo.backplane.com>

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