drm/xe/uapi: Fix code examples in xe_drm.h documentation

drm/xe/uapi: Fix code examples in xe_drm.h documentation

Fix incorrect field names and formatting in code examples:
- .num_bb_per_exec -> .width (renamed struct field
  in exec_queue_create examples)
- .num_eng_per_bb -> .num_placements (renamed struct
  field in exec_queue_create examples)
- .atomic_val -> .atomic.val (correct nested struct
  field access in madvise example)
- Remove unnecessary backslash escaping in UUID format
  string (%\08x -> %08x)
- Fix descriptive text trapped inside code-block in
  exec_queue_create doc (split into two code blocks)

v3: one more fix of split code-block in exec_queue_create doc.

Assisted-by: GitHub Copilot:claude-opus-4.6
Cc: Xin Wang <x.wang@intel.com>
Reviewed-by: Xin Wang <x.wang@intel.com>
Link: https://patch.msgid.link/20260407030046.3394004-6-shuicheng.lin@intel.com
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>