drm/vmwgfx: Release ttm memory if probe fails

The ttm mem global state was leaking if the vmwgfx driver load failed.

In case of a driver load failure we have to make sure we also release
the ttm mem global state.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105193845.258816-3-zackr@vmware.com