drm/amd/display: Do a reg update instead of set when writing ODM color format

[why]
If a set is done, DSC settings are zeroed out, leading to no DSC for the modes
that require ODM, such as 8k60.

This was a regression introduced by 5a4f26295176bbfc776c75aaf0f6dd8ccf806958.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>