drm/i915: Fix memory leaks in i915 selftests

This patch fixes memory leaks on error escapes in function fake_get_pages

Fixes: c3bfba9a ("drm/i915: Check for integer truncation on scatterlist creation")
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230414224109.1051922-1-andi.shyti@linux.intel.com