drm/amdgpu: Fix NULL ptr on driver unload due to init failure.

Problem:
When unloading due to failure amdgpu_device_fini was called twice
which was leading to NULL ptr in amdgpu_irq_disable_all.

Fix:
Call amdgpu_device_fini only once from amdgpu_driver_unload_kms.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>