drm/gem: Make drm_gem_objects_lookup() self-cleaning on failure v6

drm_gem_objects_lookup() can allocate the output array and take
references on GEM objects before it fails.

If an error happens part-way through, callers previously had to clean up
partially created results themselves. This relied on subtle and
undocumented behavior and was easy to get wrong.

Make drm_gem_objects_lookup() clean up on failure. The function now
drops any references it already took, frees the array, and sets
*objs_out to NULL before returning an error.

On success, behavior is unchanged. Existing callers remain correct and
their error cleanup paths simply do nothing when *objs_out is NULL.

v2/v3: Move partial-lookup cleanup into objects_lookup(), perform
reference dropping outside the lock, and remove reliance on __GFP_ZERO
or implicit NULL handling. (Christian)

v4: Use goto-style error handling in objects_lookup(), drop partial
references outside the lock, and simplify drm_gem_objects_lookup()
cleanup by routing failures through err_free_handles as suggested.
(Christian)

v5: Rebase on drm-misc-next, drop the ret local variable. (Christian)

v6: Drop superfluous initialization of handles. (Christian/Tvrtko)

Cc: Alex Deucher <alexander.deucher@amd.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Suggested-by: Tvrtko Ursulin <tursulin@ursulin.net>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patch.msgid.link/20260206132141.1474191-1-srinivasan.shanmugam@amd.com

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