fix for crashes provoked by UMS mode

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