drm/i915: Extract compute_partial_view()

In order to reuse the partial view for selftesting, extract the common
function for computing the view.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170110095633.6612-1-chris@chris-wilson.co.uk