drm/amdgpu: Fix NULL ptr deref in amdgpu_device_cache_switch_state()

Kaveri has no upstream bridge, therefore parent is NULL.

$ lspci -PP
...
00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Kaveri [Radeon R7 Graphics] (rev d4)

For comparison, Raphael:

$ lspci -PP
...
00:08.1 PCI bridge: Advanced Micro Devices, Inc. [AMD] Raphael/Granite Ridge Internal GPP Bridge to Bus [C:A]
...
00:08.1/0e:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Raphael (rev c5)

Fixes: 1dd2fa0e ("drm/amdgpu: Save and restore switch state")
Link: https://lore.kernel.org/amd-gfx/38fe6513-f8a9-4669-8e86-89c54c465611@gmail.com/


Reviewed-by: Candice Li <candice.li@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: John Olender <john.olender@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>