drm/vgem: drop DRM_AUTH usage from the driver

The authentication can be circumvented, by design, by using the render
node.

From the driver POV there is no distinction between primary and render
nodes, thus we can drop the token.

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527081741.14235-11-emil.l.velikov@gmail.com