drm/amdgpu: fix SDMA hdp flush engine conflict

Each of HDP flush engine should be used by one ring, correct allocate of
hdp flush engine to SDMA ring.
Correct me value of each SDMA ring, as it was cleared when init microcode.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>