amdgpu/nbio: use constant nbio_hdp_flush_reg structs.

This removes the init path as well, since the init path
just did some constant init of some structs.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>