drm: Fix potential null-ptr-deref in drm_vblank_destroy_worker()

drm: Fix potential null-ptr-deref in drm_vblank_destroy_worker()

drm_vblank_init() call drmm_add_action_or_reset() with
drm_vblank_init_release() as action. If __drmm_add_action() failed, will
directly call drm_vblank_init_release() with the vblank whose worker is
NULL. As the resule, a null-ptr-deref will happen in
kthread_destroy_worker(). Add the NULL check before calling
drm_vblank_destroy_worker().

BUG: null-ptr-deref
KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
CPU: 5 PID: 961 Comm: modprobe Not tainted 6.0.0-11331-gd465bff130bf-dirty
RIP: 0010:kthread_destroy_worker+0x25/0xb0
  Call Trace:
    <TASK>
    drm_vblank_init_release+0x124/0x220 [drm]
    ? drm_crtc_vblank_restore+0x8b0/0x8b0 [drm]
    __drmm_add_action_or_reset+0x41/0x50 [drm]
    drm_vblank_init+0x282/0x310 [drm]
    vkms_init+0x35f/0x1000 [vkms]
    ? 0xffffffffc4508000
    ? lock_is_held_type+0xd7/0x130
    ? __kmem_cache_alloc_node+0x1c2/0x2b0
    ? lock_is_held_type+0xd7/0x130
    ? 0xffffffffc4508000
    do_one_initcall+0xd0/0x4f0
    ...
    do_syscall_64+0x35/0x80
    entry_SYSCALL_64_after_hwframe+0x46/0xb0

Fixes: 5e6c2b4f9161 ("drm/vblank: Add vblank works")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101070716.9189-3-shangxiaojing@huawei.com