drm/amd/display: fix minor coding errors where dml21 phase 5 uses wrong variables

[why & how]
There is a coding error which causes incorrect variables to be assigned
in DML21 phase 5.

Reviewed-by: Austin Zheng <austin.zheng@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>