drm/xe/guc_submit: Simplify and fix diff calculation

With a u32 type, there's no need to check which one is greater: the
current is always the latest and if it's less than the previous, it's
because it wrapped: just do the unsigned calculation that will lead to
the same result, or better the correct one. It fixes an off-by-one in
the wrapped calculation, however that doesn't really matter for the
timeout calculation.

Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250513-time-wrap-v1-1-fba9a69a65c8@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>