Previous commit removed the only reason why we were allowing NULL as
a parent device. With that resolved, we can enforce nobody else does
that mistake.

With that we can drop the ugly drm_dev_set_unique workaround.

v2: rebase, silence checkpatch "== NULL" warning

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Deepak Sharma <deepak.sharma@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023163550.15211-2-emil.l.velikov@gmail.com