drm/qxl: add delayed fb operations

Due to the nature of qxl hw we cannot queue operations while in an irq
context, so we queue these operations as best we can until atomic allocations
fail, and dequeue them later in a work queue.

Daniel looked over the locking on the list and agrees it should be sufficent.

The atomic allocs use no warn, as the last thing we want if we haven't memory
to allocate space for a printk in an irq context is more printks.

Signed-off-by: Dave Airlie <airlied@redhat.com>