drm/vmwgfx: Correctly set the name of the preferred mode

Our sysfs "modes" entries were broken because our preffered mode
never had its name set correctly. This resulted in the first
entry simply being called "preferred" followed by a list of
other resolutions. Lets fix it by actually setting the name of
mode (which is its resolution). This allows one to quickly
validate the modes set by the open-vm-tools.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209161700.335611-1-zackr@vmware.com