drm/amdgpu/sdma4: drop hdp flush from wptr shadow update

The wb buffer is in system memory, not vram so the flush
is useless.

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