drm/ttm: use an operation ctx for ttm_mem_global_alloc

forward the operation context to ttm_mem_global_alloc as well, and the
ultimate goal is swapout enablement for reserved BOs

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>