drm: vmwgfx: fix information leak to userland

Structure drm_vmw_fence_rep is copied to userland with field "pad64"
uninitialized.  It leads to leaking of contents of kernel stack memory.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>