drm/i915/selftests: exercise shmem_writeback with THP

In commit:

commit 1e6decf3
Author: Hugh Dickins <hughd@google.com>
Date:   Thu Sep 2 14:54:43 2021 -0700

    shmem: shmem_writepage() split unlikely i915 THP

it looks THP + shmem_writeback was an unexpected combination, and ends up
hitting some BUG_ON, but it also looks like that is now fixed.

While the IGTs did eventually hit this(although not during pre-merge it
seems), it's likely worthwhile adding some explicit coverage for this
scenario in the shrink_thp selftest.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/4166Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210921142116.3807946-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/selftests/huge_pages.c

@@ -1589,7 +1589,8 @@ static int igt_shrink_thp(void *arg)
 	i915_gem_shrink(NULL, i915, -1UL, NULL,
 			I915_SHRINK_BOUND |
 			I915_SHRINK_UNBOUND |
-			I915_SHRINK_ACTIVE);
+			I915_SHRINK_ACTIVE |
+			I915_SHRINK_WRITEBACK);
 	if (should_swap == i915_gem_object_has_pages(obj)) {
 		pr_err("unexpected pages mismatch, should_swap=%s\n",
 		       yesno(should_swap));