drm/amd/display: Fix dmub trace event not update issue

[Why & How]
Reference to read pointer which is incorrect.
Change to reference to write pointer.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>