drm/amdgpu: hdp flush&inval should always do

This fixes Tonga vm-fault issue when running disaster
(a multiple context GL heavy tests),
We should always flush & invalidate hdp no matter vm
used or not.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <David1.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org