drm/amdkfd: Fix a building error when KFD_SUPPORT_IOMMU_V2 is turned off

The issue was accidentally introduced recently.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>