via_drm.h: move struct via_file_private definition to drivers/gpu/drm/via/via_drv.h

Fixes userspace compile error since list_head is not exported to userspace
headers.

Suggested by Emil Velikov <emil.l.velikov@gmail.com> at
https://lkml.org/lkml/2015/6/3/792



Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>