drm/i915: don't assume struct page in i915_sg_trim

If we copy all the contents of the sg across and not just the page link,
we can then also put it to work in fake_get_huge_pages and beyond.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920142707.19659-1-matthew.auld@intel.com