intel: Protect against waiting on a NULL render target bo

intel: Protect against waiting on a NULL render target bo

If we fall back to software rendering due to the render target being
absent (GPU hang or other error in creating the named target), then we
do not need to nor should we wait upon the results.

Reported-by: Magnus Kessler <Magnus.Kessler@gmx.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34656
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>