drm/ttm: Remove explicit typecasts of vm_private_data

The explicit typcasts are meaningless, so remove them.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/332899/


Signed-off-by: Christian König <christian.koenig@amd.com>