Checked the hex of atikmdag.sys, there are 4 parameters of disable dither:
TMDS_DisableDither......HDMI_DisableDither......DP_DisableDither........Embedded_DisableDither..
Run following registry to apply:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0000]
"DP_DisableDither"=dword:00000000
"Embedded_DisableDither"=dword:00000001
"TMDS_DisableDither"=dword:00000000
"HDMI_DisableDither"=dword:00000000
What the "Embedded" mean?