drm/amdgpu: remove all sh mem register modification in vm flush

Leave that at the values set during init.  No need to update
them repeatedly.

Signed-off-by: monk.liu <monk.liu@amd.com>
Signed-off-by: David Zhang <david1.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <jammy.zhou@amd.com>