drm/vgem: Stop calling drm_drv_set_unique

With the previous patch this is now redudant, the core always
sets a reasonable dev->unique string.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466499262-18717-4-git-send-email-daniel.vetter@ffwll.ch