drm/vmwgfx: Fix Legacy Display Unit

Legacy DU was broken by the referenced fixes commit because the placement
and the busy_placement no longer pointed to the same object. This was later
fixed indirectly by commit a78a8da5
("drm/ttm: replace busy placement with flags v6") in v6.9.

Fixes: 39985eea ("drm/vmwgfx: Abstract placement selection")
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Cc: <stable@vger.kernel.org> # v6.4+
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240425200700.24403-1-ian.forbes@broadcom.com