When we setup page directories and tables, we point the entries
to a to the next level scratch structure. Make this generic
by introducing a fill_page_dma which maps and flushes. We also
need 32 bit variant for legacy gens.

v2: Fix flushes and handle valleyview (Ville)
v3: Now really fix flushes (Michel, Ville)
Reviewed-by: NMichel Thierry <michel.thierry@intel.com>
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>