drm/amd/display: Add min_dcfclk_mhz field to bb overrides

Add min_dcfclk_mhz to bounding box overrides.

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>