drm/amdgpu: Fix call trace warning and hang when removing amdgpu device

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: Fix call trace warning and hang when removing amdgpu device

On GPUs with RAS enabled, below call trace and hang are observed when
shutting down device.

v2: use DRM device unplugged flag instead of shutdown flag as the check to
prevent memory wipe in shutdown stage.

[ +0.000000] RIP: 0010:amdgpu_vram_mgr_fini+0x18d/0x1c0 [amdgpu]
[ +0.000001] PKRU: 55555554
[ +0.000001] Call Trace:
[ +0.000001] <TASK>
[ +0.000002] amdgpu_ttm_fini+0x140/0x1c0 [amdgpu]
[ +0.000183] amdgpu_bo_fini+0x27/0xa0 [amdgpu]
[ +0.000184] gmc_v11_0_sw_fini+0x2b/0x40 [amdgpu]
[ +0.000163] amdgpu_device_fini_sw+0xb6/0x510 [amdgpu]
[ +0.000152] amdgpu_driver_release_kms+0x16/0x30 [amdgpu]
[ +0.000090] drm_dev_release+0x28/0x50 [drm]
[ +0.000016] devm_drm_dev_init_release+0x38/0x60 [drm]
[ +0.000011] devm_action_release+0x15/0x20
[ +0.000003] release_nodes+0x40/0xc0
[ +0.000001] devres_release_all+0x9e/0xe0
[ +0.000001] device_unbind_cleanup+0x12/0x80
[ +0.000003] device_release_driver_internal+0xff/0x160
[ +0.000001] driver_detach+0x4a/0x90
[ +0.000001] bus_remove_driver+0x6c/0xf0
[ +0.000001] driver_unregister+0x31/0x50
[ +0.000001] pci_unregister_driver+0x40/0x90
[ +0.000003] amdgpu_exit+0x15/0x120 [amdgpu]