drm/amd/display: Change viewport limit to 12 for DCN2

[WHY & HOW]
Viewport limit was set to 16 pixels due to an issue with MPO
on small viewports. This restriction does not apply and the
viewport limit can now be lowered.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>