anv: Initialize anv_bo::offset to -1

anv: Initialize anv_bo::offset to -1

Since -1 is an invalid GPU address, this lets us know whether or not we
have a valid address for a buffer.  We don't get a valid address until the
first time that buffer is used in an execbuf2 ioctl.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>