drm/amdkfd: Fix -Wunused-const-variable warning

If KFD_SUPPORT_IOMMU_V2 is not set, gcc warns:

drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_device.c:121:37: warning: ‘raven_device_info’ defined but not used [-Wunused-const-variable=]
 static const struct kfd_device_info raven_device_info = {
                                     ^~~~~~~~~~~~~~~~~

As Huang Rui suggested, Raven already has the fallback path,
so it should be out of IOMMU v2 flag.

Suggested-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>