drm/vmwgfx: Avoid cmdbuf alloc sleeping if !TASK_RUNNING

If the command buffer pool is out of space, the code waits until space is
available. However since the condition code tries to allocate a range manager
node while !TASK_RUNNING we get a kernel warning.

Avoid this by pre-allocating the mm node. This will also probably be more
efficient.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>