drm/nouveau: fix memory leak in nouveau_conn_reset()

In nouveau_conn_reset(), if connector->state is true,
__drm_atomic_helper_connector_destroy_state() will be called,
but the memory pointed by asyc isn't freed. Memory leak happens
in the following function __drm_atomic_helper_connector_reset(),
where newly allocated asyc->state will be assigned to connector->state.

So using nouveau_conn_atomic_destroy_state() instead of
__drm_atomic_helper_connector_destroy_state to free the "old" asyc.

Here the is the log showing memory leak.

unreferenced object 0xffff8c5480483c80 (size 192):
  comm "kworker/0:2", pid 188, jiffies 4294695279 (age 53.179s)
  hex dump (first 32 bytes):
    00 f0 ba 7b 54 8c ff ff 00 00 00 00 00 00 00 00  ...{T...........
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000005005c0d0>] kmem_cache_alloc_trace+0x195/0x2c0
    [<00000000a122baed>] nouveau_conn_reset+0x25/0xc0 [nouveau]
    [<000000004fd189a2>] nouveau_connector_create+0x3a7/0x610 [nouveau]
    [<00000000c73343a8>] nv50_display_create+0x343/0x980 [nouveau]
    [<000000002e2b03c3>] nouveau_display_create+0x51f/0x660 [nouveau]
    [<00000000c924699b>] nouveau_drm_device_init+0x182/0x7f0 [nouveau]
    [<00000000cc029436>] nouveau_drm_probe+0x20c/0x2c0 [nouveau]
    [<000000007e961c3e>] local_pci_probe+0x47/0xa0
    [<00000000da14d569>] work_for_cpu_fn+0x1a/0x30
    [<0000000028da4805>] process_one_work+0x27c/0x660
    [<000000001d415b04>] worker_thread+0x22b/0x3f0
    [<0000000003b69f1f>] kthread+0x12f/0x150
    [<00000000c94c29b7>] ret_from_fork+0x3a/0x50

Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>