drm/amdgpu: fix the nullptr issue when reenter GPU recovery

in single gpu system, if driver reenter gpu recovery,
amdgpu_device_lock_adev will return false, but hive is
nullptr now.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>