drm/amdgpu/hdp6: use memcfg register to post the write for HDP flush

drm/amdgpu/hdp6: use memcfg register to post the write for HDP flush

commit ca28e80abe4219c8f1a2961ae05102d70af6dc87 upstream.

Reading back the remapped HDP flush register seems to cause
problems on some platforms. All we need is a read, so read back
the memcfg register.

Fixes: abe1cbaec6cf ("drm/amdgpu/hdp6.0: do a posting read when flushing HDP")
Reported-by: Alexey Klimov <alexey.klimov@linaro.org>
Link: https://lists.freedesktop.org/archives/amd-gfx/2025-April/123150.html
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4119
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3908
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 84141ff615951359c9a99696fd79a36c465ed847)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>