drm/amd/display: Set one 4:2:0-related PPS field as recommended by DSC spec

[why]
'second_line_offset_adj' was mistakenly left at zero, even though DSC spec
v1.2a recommends setting this field to 512 for 4:2:0.

[how]
Set 'second_line_offset_adj' to 512 for 4:2:0 and leave at zero otherwise

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>