drm/ttm: Remove the struct ttm_backup abstraction

drm/ttm: Remove the struct ttm_backup abstraction

The abstraction was previously added to support separate
ttm_backup implementations.

However with the current implementation casting from a
struct file to a struct ttm_backup, we run into trouble since
struct file may have randomized the layout and gcc complains.

Remove the struct ttm_backup abstraction

Cc: dri-devel@lists.freedesktop.org
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Kees Cook <kees@kernel.org>
Closes: https://lore.kernel.org/dri-devel/9c8dbbafdaf9f3f089da2cde5a772d69579b3795.camel@linux.intel.com/T/#mb153ab9216cb813b92bdeb36f391ad4808c2ba29
Suggested-by: Christian König <christian.koenig@amd.com>
Fixes: 70d645deac98 ("drm/ttm: Add helpers for shrinking")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250502130014.3156-1-thomas.hellstrom@linux.intel.com