drm/i915/gtt: Introduce struct i915_page_dma

All our paging structures have struct page and dma address
for that page.

Add struct for page/dma address pairs and use it to make
the setup and teardown for different paging structures
identical.

Include the page directory offset also in the struct for legacy
gens. Rename it to clearly point out that it is offset into the
ggtt.

v2: Add comment about ggtt_offset (Michel)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>