checker for linear surfaces

There are too many strange corner cases triggered in old userspace
drivers out there to that it's nearly impossible to not break some
obscure app.

Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>