Revert "drm/amdgpu: Use generic hdp flush function"

This reverts commit 18a878fd.

Revert this temporarily to make it easier to fix a regression
in the HDP handling.

Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>