drm/amd/display: Change to IMMEDIATE mode from FRAME mode

[Why]
Change in DCN10 to use IMMEDIATE_UPDATE mode for AFMT is not
reflected in DCN30 as it uses VPG.

[How]
Use IMMEDIATE_UPDATE mode for DCN30 in VPG.

Signed-off-by: Judy Cai <HuiYi.Cai@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>