gbm: Use unsigned for BO offset getter

gbm: Use unsigned for BO offset getter

The actual offset returned is uint32_t, however int64_t was used as the
return type from gbm_bo_get_offset to allow negative returns to signal
errors to the caller.

In case of an error getting the offset, the user will also be unable to
get the handle/FD, and thus have nothing to offset into. This means that
returning 0 as an error value is harmless, allowing us to change the
return type to uint32_t in order to avoid signed/unsigned confusion in
callers.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>