drm/amd/display: Renoir chroma viewport WA Read the correct register

[why]
Before we were reading registers specific to luma size, which caused a black line
to appear on the screen from time to time, as although the luma row height
is generally the same as the chroma row height for the video case, it will sometimes
be one more

[how]
Read the register specific for the chroma size

Signed-off-by: Joseph Gravenor <joseph.gravenor@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>