drm/amdgpu: Add the HDP flush support for Navi

The HDP flush support code was missing in the nbio and nv files.

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>