drm/atomic: Unref duplicated drm_atomic_state in drm_atomic_helper_resume()

Kmemleak reported memory leak after suspend and resume:
unreferenced object 0xffffffc0e31d8880 (size 128):
  comm "bash", pid 181, jiffies 4294763583 (age 24.694s)
  hex dump (first 32 bytes):
    01 00 00 00 00 00 00 00 00 20 a2 eb c0 ff ff ff  ......... ......
    01 00 00 00 00 00 00 00 80 87 1d e3 c0 ff ff ff  ................
  backtrace:
    [<ffffffc00034bb64>] __save_stack_trace+0x48/0x6c
    [<ffffffc00034c244>] create_object+0x138/0x254
    [<ffffffc0009dd218>] kmemleak_alloc+0x58/0x8c
    [<ffffffc000346de4>] kmem_cache_alloc_trace+0x188/0x254
    [<ffffffc0005af4c0>] drm_atomic_state_alloc+0x3c/0x88
    [<ffffffc000591f0c>] drm_atomic_helper_duplicate_state+0x28/0x158
    [<ffffffc000592098>] drm_atomic_helper_suspend+0x5c/0xf0

Problem here is that we are duplicating the drm_atomic_state in
drm_atomic_helper_suspend(), but not unreference it in the resume path.

Fixes: 14942760 ("drm/atomic-helper: Implement subsystem-level suspend/resume")
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171009064641.15174-1-jeffy.chen@rock-chips.com
Fixes: 0853695c ("drm: Add reference counting to drm_atomic_state")
Cc: <stable@vger.kernel.org> # v4.10+