drm/i915/dmabuf: Use scatterlist for_each_sg API

Update open coded for loop to use the standard scatterlist
for_each_sg API.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221028155029.494736-4-matthew.auld@intel.com