drm/tests: helpers: Avoid a driver uaf

In the Linux kernel, the following vulnerability has been resolved:

drm/tests: helpers: Avoid a driver uaf

when using __drm_kunit_helper_alloc_drm_device() the driver may be
dereferenced by device-managed resources up until the device is
freed, which is typically later than the kunit-managed resource code
frees it. Fix this by simply make the driver device-managed as well.

In short, the sequence leading to the UAF is as follows:

INIT:
Code allocates a struct device as a kunit-managed resource.
Code allocates a drm driver as a kunit-managed resource.
Code allocates a drm device as a device-managed resource.

EXIT:
Kunit resource cleanup frees the drm driver
Kunit resource cleanup puts the struct device, which starts a
device-managed resource cleanup
device-managed cleanup calls drm_dev_put()
drm_dev_put() dereferences the (now freed) drm driver -> Boom.

Related KASAN message:
[55272.551542] ==================================================================
[55272.551551] BUG: KASAN: slab-use-after-free in drm_dev_put.part.0+0xd4/0xe0 [drm]
[55272.551603] Read of size 8 at addr ffff888127502828 by task kunit_try_catch/10353

[55272.551612] CPU: 4 PID: 10353 Comm: kunit_try_catch Tainted: G U N 6.5.0-rc7+ #155
[55272.551620] Hardware name: ASUS System Product Name/PRIME B560M-A AC, BIOS 0403 01/26/2021
[55272.551626] Call Trace:
[55272.551629] <TASK>
[55272.551633] dump_stack_lvl+0x57/0x90
[55272.551639] print_report+0xcf/0x630
[55272.551645] ? _raw_spin_lock_irqsave+0x5f/0x70
[55272.551652] ? drm_dev_put.part.0+0xd4/0xe0 [drm]
[55272.551694] kasan_report+0xd7/0x110
[55272.551699] ? drm_dev_put.part.0+0xd4/0xe0 [drm]
[55272.551742] drm_dev_put.part.0+0xd4/0xe0 [drm]
[55272.551783] devres_release_all+0x15d/0x1f0
[55272.551790] ? __pfx_devres_release_all+0x10/0x10
[55272.551797] device_unbind_cleanup+0x16/0x1a0
[55272.551802] device_release_driver_internal+0x3e5/0x540
[55272.551808] ? kobject_put+0x5d/0x4b0
[55272.551814] bus_remove_device+0x1f1/0x3f0
[55272.551819] device_del+0x342/0x910
[55272.551826] ? __pfx_device_del+0x10/0x10
[55272.551830] ? lock_release+0x339/0x5e0
[55272.551836] ? kunit_remove_resource+0x128/0x290 [kunit]
[55272.551845] ? __pfx_lock_release+0x10/0x10
[55272.551851] platform_device_del.part.0+0x1f/0x1e0
[55272.551856] ? _raw_spin_unlock_irqrestore+0x30/0x60
[55272.551863] kunit_remove_resource+0x195/0x290 [kunit]
[55272.551871] ? _raw_spin_unlock_irqrestore+0x30/0x60
[55272.551877] kunit_cleanup+0x78/0x120 [kunit]
[55272.551885] ? __kthread_parkme+0xc1/0x1f0
[55272.551891] ? __pfx_kunit_try_run_case_cleanup+0x10/0x10 [kunit]
[55272.551900] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10 [kunit]
[55272.551909] kunit_generic_run_threadfn_adapter+0x4a/0x90 [kunit]
[55272.551919] kthread+0x2e7/0x3c0
[55272.551924] ? __pfx_kthread+0x10/0x10
[55272.551929] ret_from_fork+0x2d/0x70
[55272.551935] ? __pfx_kthread+0x10/0x10
[55272.551940] ret_from_fork_asm+0x1b/0x30
[55272.551948] </TASK>

[55272.551953] Allocated by task 10351:
[55272.551956] kasan_save_stack+0x1c/0x40
[55272.551962] kasan_set_track+0x21/0x30
[55272.551966] __kasan_kmalloc+0x8b/0x90
[55272.551970] __kmalloc+0x5e/0x160
[55272.551976] kunit_kmalloc_array+0x1c/0x50 [kunit]
[55272.551984] drm_exec_test_init+0xfa/0x2c0 [drm_exec_test]
[55272.551991] kunit_try_run_case+0xdd/0x250 [kunit]
[55272.551999] kunit_generic_run_threadfn_adapter+0x4a/0x90 [kunit]
[55272.552008] kthread+0x2e7/0x3c0
[55272.552012] ret_from_fork+0x2d/0x70
[55272.552017] ret_from_fork_asm+0x1b/0x30

[55272.552024] Freed by task 10353:
[55272.552027] kasan_save_stack+0x1c/0x40
[55272.552032] kasan_set_track+0x21/0x30
[55272.552036] kasan_save_free_info+0x27/0x40
[55272.552041] __kasan_slab_free+0x106/0x180
[55272.552046] slab_free_freelist_hook+0xb3/0x160
[55272.552051] __kmem_cache_free+0xb2/0x290
[55272.552056] kunit_remove_resource+0x195/0x290 [kunit]
[55272.552064] kunit_cleanup+0x7
---truncated---