drm/vmwgfx: Take the ttm lock around the dirty ioctl

This makes sure noone accesses the fifo while it's taken down using the
dirty ioctl.
Also make sure all workqueues are idled before the fifo is taken down.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>