swr: support NULL-resources

swr: support NULL-resources

It's legal for a buffer-object to have a NULL-resource, but let's just
skip over it, as there's nothing to do.

This patch switches the order of the conditionals in swr_update_derived,
so the logic becomes a bit more straight forward:

if (is_user_buffer)
   ...
else if (resource)
   ...
else
   ...

...instead of this:

if (!is_user_buffer)
   if (resource)
      ...
   else
      ...
else
   ...

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Alok Hota <alok.hota@intel.com>