The truncation isn't being programmed if the truncation
depth is set to 2, it causes an issue with dce11.2 asic
using 6bit eDP panel. It required to truncate 12:10 in order to
perform spatial dither 10:6.

This change will allow 12:10 truncation to be enabled.
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>