drm/i915/gem: drop nth_page() usage within SG entry

drm/i915/gem: drop nth_page() usage within SG entry

It's no longer required to use nth_page() when iterating pages within a
single SG entry, so let's drop the nth_page() usage.

Link: https://lkml.kernel.org/r/20250901150359.867252-27-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>