drm/vmwgfx: Fix an invalid read

vmw_move assumed that buffers to be moved would always be
vmw_buffer_object's but after introduction of new placement for mob
pages that's no longer the case.
The resulting invalid read didn't have any practical consequences
because the memory isn't used unless the object actually is a
vmw_buffer_object.
Fix it by moving the cast to the spot where the results are used.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Fixes: f6be2326 ("drm/vmwgfx: Introduce a new placement for MOB page tables")
Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Tested-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318174332.440068-2-zack@kde.org