drm/amdkfd: remove set but not used variable 'top_dev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdkfd/kfd_iommu.c: In function kfd_iommu_device_init:
drivers/gpu/drm/amd/amdkfd/kfd_iommu.c:65:30: warning: variable top_dev set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 1ae99eab ("drm/amdkfd: Initialize HSA_CAP_ATS_PRESENT capability in topology codes")
Signed-off-by: zhengbin <zhengbin13@huawei.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>