drm/amd/display: Modify Vmin default value

Fine tune the Vmin clock value

Reviewed-by: Robin Chen <robin.chen@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Max Tseng <max.tseng@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>