drm/amdgpu/umsch: reinitialize write pointer in hw init

Otherwise the old one will be used during GPU reset.
That's not expected.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>