drm/amdgpu/hdp5.0: do a posting read when flushing HDP

[ Upstream commit cf424020 ]

Need to read back to make sure the write goes through.

Cc: David Belanger <david.belanger@amd.com>
Reviewed-by: Frank Min <frank.min@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>