drm/xe: Stop ignoring errors from xe_ttm_stolen_mgr_init()

drm/xe: Stop ignoring errors from xe_ttm_stolen_mgr_init()

[ Upstream commit ff57025c358603555f1e0ae0d50282a460433594 ]

Make sure to differentiate normal behavior, e.g. there's no stolen, from
allocation errors or failure to initialize lower layers.

Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250213192909.996148-5-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>