drm/amdgpu/hdp5: use memcfg register to post the write for HDP flush

drm/amdgpu/hdp5: use memcfg register to post the write for HDP flush

commit 0e33e0f339b91eecd9558311449a3d1e728722d4 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: cf424020e040 ("drm/amdgpu/hdp5.0: 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 a5cb344033c7598762e89255e8ff52827abb57a4)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>