Documentation/gpu/drm-mm: Add THP paragraph to GEM mapping section

Documentation/gpu/drm-mm: Add THP paragraph to GEM mapping section

Add a paragraph to the GEM Objects Creation section about the
drm_gem_huge_mnt_create() helper and to the GEM objects mapping
section explaining how transparent huge pages are handled by GEM.

v4:
- fix wording after huge_pages handler removal

v6:
- fix wording after map_pages handler removal

v11:
- mention drm_gem_huge_mnt_create() helper
- add Boris and Maíra R-bs

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patch.msgid.link/20251205182231.194072-11-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>