drm/amdgpu: fix kernel hang when starting VNC server

After starting VNC server or running CTS test, kernel will hang and
can see below call trace:

[961816] INFO: task khugepaged:42 blocked for more than 120 seconds.
[968581]       Tainted: G           OE   4.13.0 #1
[973495] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables
          this message.
[980962] khugepaged      D    0    42      2 0x00000000
[980967] Call Trace:
[980977]  __schedule+0x28d/0x890
[980982]  schedule+0x36/0x80
[980986]  rwsem_down_read_failed+0x139/0x1c0
[980991]  ? update_curr+0x100/0x1c0
[981004]  call_rwsem_down_read_failed+0x18/0x30
[981007]  down_read+0x20/0x40
[981012]  khugepaged_scan_mm_slot+0x78/0x1ac0
[981018]  ? __switch_to+0x23e/0x4a0
[981022]  ? finish_task_switch+0x79/0x240
[981026]  khugepaged+0x146/0x480
[981031]  ? remove_wait_queue+0x60/0x60
[981035]  kthread+0x109/0x140
[981037]  ? khugepaged_scan_mm_slot+0x1ac0/0x1ac0
[981039]  ? kthread_park+0x60/0x60
[981044]  ret_from_fork+0x25/0x30

After checking code and found 'commit b72cf4fc ("drm/amdgpu: move
taking mmap_sem into get_user_pages v2")' forget to drop one case of
up_read.

Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>