drm/xe: Set survivability mode before heci init

drm/xe: Set survivability mode before heci init

Commit d40f275d96e8 ("drm/xe: Move survivability entirely to xe_pci")
tried to follow the logic: initialize everything needed and if
everything succeeds, set the flag that it's enabled. While it fixed some
corner cases of those calls failing, it was wrong for setting the flag
after the call to xe_heci_gsc_init(): that function does a different
initialization for survivability mode.

Fix that and add comments about this being done on purpose.

Suggested-by: Riana Tauro <riana.tauro@intel.com>
Fixes: d40f275d96e8 ("drm/xe: Move survivability entirely to xe_pci")
Reviewed-by: Riana Tauro <riana.tauro@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250314-fix-survivability-v5-2-fdb3559ea965@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>