drm/ttm: Don't shadow the operation context

ttm_bo_swapout() shadows the ttm operation context which may cause
major confusion in driver callbacks when swapping out !TTM_PL_SYSTEM
memory. Fix this by reusing the operation context argument to
ttm_bo_swapout().

Cc: "Christian König" <christian.koenig@amd.com>
Cc: Roger He <Hongbo.He@amd.com>
Cc: <dri-devel@lists.freedesktop.org>
Cc: <intel-gfx@lists.freedesktop.org>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Christian König <ckoenig.leichtzumerken@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230626091450.14757-3-thomas.hellstrom@linux.intel.com