drm/ttm: add new api ttm_device_prepare_hibernation()

This new api is used for hibernation to move GTT BOs to shmem after
VRAM eviction. shmem will be flushed to swap disk later to reduce
the system memory usage for hibernation.

Signed-off-by: Samuel Zhang <guoqing.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250710062313.3226149-2-guoqing.zhang@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>