drm/vgem: create a render node for vgem

VGEM doesn't do anything modeset specific, so in a way exposing a
primary node is 'wrong'. At the same time, we extensively use if for
creating dumb buffers, fences, prime fd <> handle imports/exports.

To the point that we explicitly annotate the vgem fence ioctls as
DRM_RENDER_ALLOW and have an IGT test which opens the render node.

close(drm_open_driver_render(DRIVER_VGEM))

Better late than never, let's flip the switch.

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026120647.7528-1-emil.l.velikov@gmail.com