hdp read cache is removed in aldebaran. don't issue
an mmio write or write data packet to hardware.

v2: rebase
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>