drm/xe: Move suballocator init to after display init

No allocations should be done before we have had a chance to preserve
the display fb.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210083111.230484-4-dev@lankhorst.se


Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>