sna: Be defensive when applying size restrictions to bo

sna: Be defensive when applying size restrictions to bo

The assumption that all paths prevalidate the restrictions upon creation
the bo are false. Some important paths try to force the bo creation in
order to meet client expectations (e.g. DRI). So we are faced with
impossible requests which must fail, so make sure we do report those
failures.

Bugzilla: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1289049
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>