drm/xe: Clear all snapshot members after deleting coredump

It's not strictly needed to clear right now, but this prevents bugs
from dangling pointers.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221133024.898315-2-maarten.lankhorst@linux.intel.com