drm/vmwgfx: move the require_exist handling together

Move the render_client hunk for require_exist alongside the rest.
Keeping all the reasons why an existing object is needed, in a single
place makes it easier to follow.

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>