drm/xe: Fix xe_pt_stage_bind_walk kerneldoc

drm/xe: Fix xe_pt_stage_bind_walk kerneldoc

The structure was missing a proper kerneldoc header and once
that was added a number of typos and errors became obvious.
Fix those.

Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
Closes: https://lore.kernel.org/intel-xe/x53tcs5bjldw6lcorjemuheklxcmepdvr2u7lvt3hpqrzqoc4h@nsu6hs25taqj/
Fixes: b2d4b03b03a7 ("drm/xe: Make the PT code handle placement per PTE rather than per vma / range")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250402122924.25526-1-thomas.hellstrom@linux.intel.com