EGL's native display type is not particularly well defined; in
gl-renderer we get around this by always treating it as a void *, since
for all the platforms we care about it's a pointer - gbm_device,
wl_display, or Display *.

The surfaceless platform doesn't care what the native display is (since
it doesn't have one by definition), so just use NULL instead of what may
be either NULL or 0 depending on environmental factors.

Signed-off-by: Daniel Stone <daniels@collabora.com>