drm/amdgpu: fix NULL pointer panic of emit_gds_switch

[  338.384770] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  338.384817] IP: [<          (null)>]           (null)
[  338.385505] RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
[  338.385950] Call Trace:
[  338.385993]  [<ffffffffa05d2313>] ? amdgpu_vm_flush+0x283/0x400 [amdgpu]
[  338.386025]  [<ffffffff811818d3>] ? printk+0x4d/0x4f
[  338.386074]  [<ffffffffa05d4906>] amdgpu_ib_schedule+0x4a6/0x4d0 [amdgpu]
[  338.386140]  [<ffffffffa0673e54>] amdgpu_job_run+0x64/0x180 [amdgpu]
[  338.386203]  [<ffffffffa0672e09>] amd_sched_main+0x2e9/0x4a0 [amdgpu]
[  338.386232]  [<ffffffff810bfce0>] ? prepare_to_wait_event+0x110/0x110
[  338.386295]  [<ffffffffa0672b20>] ? amd_sched_select_entity+0xe0/0xe0 [amdgpu]
[  338.386327]  [<ffffffff8109b423>] kthread+0xd3/0xf0
[  338.386349]  [<ffffffff8109b350>] ? kthread_park+0x60/0x60
[  338.386376]  [<ffffffff817e1ee5>] ret_from_fork+0x25/0x30
[  338.386401] Code:  Bad RIP value.
[  338.386420] RIP  [<          (null)>]           (null)
[  338.386443]  RSP <ffffc90001bd7d40>
[  338.386458] CR2: 0000000000000000
[  338.398508] ---[ end trace 4c66fcdc74b9a0a2 ]---

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>