drm/amd/display: Restore rptr/wptr for DMCUB as workaround

[Why]
States may be desync after resume.

[How]
Sync sw state with hw state.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: JinZe.Xu <JinZe.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>