drm/amd/display: Reset OUTBOX0 r/w pointer on DMUB reset

[Why & How]
We missed resetting OUTBOX0 mailbox r/w pointer on DMUB reset.
Fix it.

Fixes: 6ecf9773

 ("drm/amd/display: Fix DMUB outbox trace in S4 (#4465)")
Signed-off-by: Cruise Hung <Cruise.Hung@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>