drm/amdgpu: add HDP flush dummy for UVD 6/7

The UVD firmware doesn't seem to like the HDP flush here.

This worked for years without HDP flush, so just skip it.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>