ttm: Clear VPO_UNMANAGED on pages allocated using vm_page_alloc_contig()

... and set PG_FICTITIOUS.

The patch comes from kib@'s deviant2 repository, commit
00eaa3d08e25e91926db172cdf9910608d8702fc.
radeon: Register one large fictitious range once

... instead of many small ranges. This is required because the number of
registered ranges is limited to 8 at the moment.