drm/xe: Map initial FB at the same place in GGTT too

I saw a flicker when booting xe, and it's very likely that the original
FB was not mapped at the same place when inheriting, fix it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>