drm/amd/display: Use 1/8th DSC target bitrate precision for N4:2:2 and 4:2:0 formats

[why]
On at least some of the devices (e.g. Realtek scaler) we get a black screen if 1/16th
precision is used.

[how]
Work around it by reducing precision to 1/8th for N4:2:2 and 4:2:0 color formats. This
is a safe workaround and would have a very mild impact on the quality.
The issue is still to be root-caused and fixed correctly.

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