drm/amd/display: Update 709 gamma to 2.222 as stated in the standerd

[WHY]
    Previously driver use gamma 2.2 for 709 color space,
    but the standard is to use gamma of 2.222

[HOW]
    Change it gamma to 2.222

Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Nawwar Ali <nawwar.ali@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>