drm/ttm: isolate dma data from ttm_tt V4

Move dma data to a superset ttm_dma_tt structure which herit
from ttm_tt. This allow driver that don't use dma functionalities
to not have to waste memory for it.

V2 Rebase on top of no memory account changes (where/when is my
   delorean when i need it ?)
V3 Make sure page list is initialized empty
V4 typo/syntax fixes

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>