drm/gem: Add huge tmpfs mountpoint helpers

drm/gem: Add huge tmpfs mountpoint helpers

Add the drm_gem_huge_mnt_create() and drm_gem_get_huge_mnt() helpers
to avoid code duplication in the i915, V3D, Panfrost and Panthor
drivers. The former creates and mounts a dedicated huge tmpfs
mountpoint, for the lifetime of a DRM device, used at GEM object
initialization. The latter retrieves the dedicated huge tmpfs
mountpoint used by a DRM device.

The next commits will port drivers to these helpers.

v3:
- store huge tmpfs mountpoint in drm_device

v4:
- return 0 in builds with CONFIG_TRANSPARENT_HUGEPAGE=n
- return 0 when huge_mnt already exists
- use new vfs_parse_fs_string() helper

v5:
- remove warning on !dev->huge_mnt and reset to NULL on free
- inline drm_gem_huge_mnt_create() to remove func from text and avoid
  calls in builds with CONFIG_TRANSPARENT_HUGEPAGE=n
- compile out drm_device's huge_mnt field in builds with
  CONFIG_TRANSPARENT_HUGEPAGE=n
- add drm_gem_has_huge_mnt() helper

v6:
- move huge_mnt doc into ifdef'd section
- either inline or export drm_gem_huge_mnt_create()

v7:
- include <drm/drm_device.h> in drm_gem.h

v9:
- replace drm_gem_has_huge_mnt() by drm_gem_get_huge_mnt()

v11:
- doc fixes
- add Boris and Maíra R-bs

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patch.msgid.link/20251205182231.194072-5-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>