drm/amdgpu/nbio7.4: use original HDP_FLUSH bits

The extended bits were not available for use on vega20 and
presumably arcturus as well.

Fixes: a0f9f854

 ("drm/amdgpu/nbio7.4: don't use GPU_HDP_FLUSH bit 12")
Reviewed-and-tested-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>