drm/ttm: move swapout logic around v3

Move the iteration of the global lru into the new function
ttm_global_swapout() and use that instead in drivers.

v2: consistently return int
v3: fix build fail

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/424008/