con may be large
enough for us to borrow it for duration of load detection.

Found by assert_fb_bound_for_plane().

Reported-by: Knut Petersen <Knut_Petersen@t-online.de>
References: https://bugs.freedesktop.org/show_bug.cgi?id=36246
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>
"7t[–$x