bad argument if(tmp)... in check_free_hole

fix oops: kernel BUG at drivers/gpu/drm/drm_mm.c:305!

[airlied: excellent, this was my task for today].
Signed-off-by: Krzysztof Kolasa <kkolasa@winsoft.pl>
Reviewed-by: Chris wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>