drm/vgem: Set unique to "vgem"

Since there's only one global instance ever we don't need to have
anything fancy. Stops a WARNING in the get_unique ioctl that the
unique name isn't set.

Cc: <stable@vger.kernel.org> # 4.1+ only
Reportedy-and-tested-by: Fabio Coatti <fabio.coatti@gmail.com>
Cc: Fabio Coatti <fabio.coatti@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>