drm/amdgpu: Fix blocking in RCU critical section(v2)

In RCU read-side critical sections, blocking or sleeping is prohibited.

v2: Unlock RCU for the code path where result==NULL. (David Zhou)
    Update subject

Tested-by and reported by: Dave Airlie <airlied@redhat.com>

[  141.965723] =============================
[  141.965724] WARNING: suspicious RCU usage
[  141.965726] 4.12.0-rc7 #221 Not tainted
[  141.965727] -----------------------------
[  141.965728] /home/airlied/devel/kernel/linux-2.6/include/linux/rcupdate.h:531
Illegal context switch in RCU read-side critical section!
[  141.965730]
               other info that might help us debug this:

[  141.965731]
               rcu_scheduler_active = 2, debug_locks = 0
[  141.965732] 1 lock held by amdgpu_cs:0/1332:
[  141.965733]  #0:  (rcu_read_lock){......}, at: [<ffffffffa01a0d07>]
amdgpu_bo_list_get+0x0/0x109 [amdgpu]
[  141.965774]
               stack backtrace:
[  141.965776] CPU: 6 PID: 1332 Comm: amdgpu_cs:0 Not tainted 4.12.0-rc7 #221
[  141.965777] Hardware name: To be filled by O.E.M. To be filled by
O.E.M./M5A97 R2.0, BIOS 2603 06/26/2015
[  141.965778] Call Trace:
[  141.965782]  dump_stack+0x68/0x92
[  141.965785]  lockdep_rcu_suspicious+0xf7/0x100
[  141.965788]  ___might_sleep+0x56/0x1fc
[  141.965790]  __might_sleep+0x68/0x6f
[  141.965793]  __mutex_lock+0x4e/0x7b5
[  141.965817]  ? amdgpu_bo_list_get+0xa4/0x109 [amdgpu]
[  141.965820]  ? lock_acquire+0x125/0x1b9
[  141.965844]  ? amdgpu_bo_list_set+0x464/0x464 [amdgpu]
[  141.965846]  mutex_lock_nested+0x16/0x18
[  141.965848]  ? mutex_lock_nested+0x16/0x18
[  141.965872]  amdgpu_bo_list_get+0xa4/0x109 [amdgpu]
[  141.965895]  amdgpu_cs_ioctl+0x4a0/0x17dd [amdgpu]
[  141.965898]  ? radix_tree_node_alloc.constprop.11+0x77/0xab
[  141.965916]  drm_ioctl+0x264/0x393 [drm]
[  141.965939]  ? amdgpu_cs_find_mapping+0x83/0x83 [amdgpu]
[  141.965942]  ? trace_hardirqs_on_caller+0x16a/0x186

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>