drm/amd/display: Add missing lines of code in dc.c

[Why & How]
A critial part of "drm/amd/display: Fix windowed MPO video with ODM
combine for DCN32" is lost during promotion to upstream. This patch
addes the code back to dc.c.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>