drm/i915/gtt: Introduce i915_page_dir_dma_addr

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>