fix for crashes provoked by UMS mode

[Ilija Hadzic]

At Dave's request I ran some regression tests on my CS-refactoring
patches [1] against old UMS userspace. The tests have revealed
that the current kernel can be provoked into crashing in UMS mode
(and the problem is unrelated to refactoring of CS code; it has
been here before).

The following patches will fix the problem that I discovered
while testing (and they should probably go to drm-fixes branch).
First two patches are the fix for the acute problem, while
the third patch is a fix for an incidental finding uncovered
while debugging the problem.


[1] http://lists.freedesktop.org/archives/dri-devel/2013-January/032814.html