drm/vmwgfx: Stop using raw ttm_buffer_object's

Various bits of the driver used raw ttm_buffer_object instead of the
driver specific vmw_bo object. All those places used to duplicate
the mapped bo caching policy of vmw_bo.

Instead of duplicating all of that code and special casing various
functions to work both with vmw_bo and raw ttm_buffer_object's unify
the buffer object handling code.

As part of that work fix the naming of bo's, e.g. insted of generic
backup use 'guest_memory' because that's what it really is.

All of it makes the driver easier to maintain and the code easier to
read. Saves 100+ loc as well.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230131033542.953249-9-zack@kde.org