drm/buddy: Add kernel-doc for allocator structures and flags

drm/buddy: Add kernel-doc for allocator structures and flags

Add missing kernel-doc for GPU buddy allocator flags,
gpu_buddy_block, and gpu_buddy. The documentation covers block
header fields, allocator roots, free trees, and allocation flags
such as RANGE, TOPDOWN, CONTIGUOUS, CLEAR, and TRIM_DISABLE.
Private members are marked with kernel-doc private markers
and documented with regular comments.

No functional changes.

v2:
- Corrected GPU_BUDDY_CLEAR_TREE and GPU_BUDDY_DIRTY_TREE index
  values (Arun)
- Rebased after DRM buddy allocator moved to drivers/gpu/
- Updated commit message

v3:
- Document reserved bits 8:6 in header layout (Arun)
- Fix checkpatch warning

Cc: Christian König <christian.koenig@amd.com>
Cc: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Sanjay Yadav <sanjay.kumar.yadav@intel.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patch.msgid.link/20260212092527.718455-5-sanjay.kumar.yadav@intel.com