drm/xe/lrc: give start_seqno a better default

If looking at the initial engine dump we should expect this to match
XE_FENCE_INITIAL_SEQNO - 1.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>