The legacy mode mm switch and the execlist context assignment
needs dma address for the page directories.
Introduce a function that encapsulates the scratch_pd dma
fallback if no pd is found.
v2: Rebase, s/ring/req
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com> Reviewed-by: Michel Thierry <michel.thierry@intel.com> (v1) Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>