drm/amd/display: Allow truncation to 10 bits

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>