drm/xe/hmm: Style- and include fixes

Add proper #ifndef around the xe_hmm.h header, proper spacing
and since the documentation mostly follows kerneldoc format,
make it kerneldoc. Also prepare for upcoming -stable fixes.

Fixes: 81e058a3 ("drm/xe: Introduce helper to populate userptr")
Cc: Oak Zeng <oak.zeng@intel.com>
Cc: <stable@vger.kernel.org> # v6.10+
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Matthew Brost &lt;Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250304173342.22009-2-thomas.hellstrom@linux.intel.com