drm/amdgpu/sdma5.2: use legacy HDP flush for SDMA2/3

This avoids a potential conflict with firmwares with the newer
HDP flush mechanism.

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