Out scatterlist utility routines can be pulled out of i915_gem.h for a
bit more decluttering.

v2: Push I915_GTT_PAGE_SIZE out of i915_scatterlist itself and into the
caller.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMatthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528092956.14910-9-chris@chris-wilson.co.uk