drm/i915/gtt: Use macros to access dma mapped pages

Make paging structure type agnostic *_px macros to access
page dma struct, the backing page and the dma address.

This makes the code less cluttered on internals of
i915_page_dma.

v2: Superfluous const -> nonconst removed
v3: Rebased

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>