drm: Allow also control clients to check the drm version

This should be harmless.
Vmware will, due to old infrastructure reasons, be using a privileged
control client to supply GUI layout information rather than obtaining
it from the device. That control client will be needing access to DRM
version information.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Acked-by: David Herrmann <dh.herrmann@gmail.com>