drm/amd/amdgpu: handle return value of amdgpu_driver_load_kms

if guest driver failed to enter full GPU access, amdgpu_driver_load_kms
will unload kms and free dev->dev_private, drm_dev_register would access
null pointer. Driver will enter an error state and can't be unloaded.

Signed-off-by: Liu ChengZhe <ChengZhe.Liu@amd.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>