drm/amd/powerplay: issue proper hdp flush for table transferring

Guard the content consistence between the view of GPU and CPU
during the table transferring.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>