drm/i915: move more pte encoding to pte encode

In order to handle differences in pte encoding between architectures it
is desirable to have one helper function, pte_encode, do it all for us.
As such, this commit moves the code around so we're in good shape to do
that.

Luckily the ppgtt pte and the ggtt pte look very similar.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>