drm/vmwgfx: Switch the internal BO's to ttm_bo_type_kernel

There's never a need to access our internal kernel bo's from
user-space. Those objects are used exclusively for internal
support to guest backed surfaces (in otable setup and mob
page tables) and there's no need to have them be of device
type, i.e. mmappable from user-space.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105193845.258816-6-zackr@vmware.com