Another shot at fixing the drm suballocator lockdep splat

Another attempt that follows the locking design, that tries hard to avoid disabling irqs during the potentially lengthy drm_mm allocation.

I couldn't repro the splat on my system for some reason, so please @mbrost could you check whether this fixes the issue? If not we could as well remove the sa manager lock and irq disabling avoidance in favour of the waitqueue lock.