drm/amd/display: Update min dcfclk

[Why]
NV12 has lower min dcfclk

[How]
Add update in update_bounding_box

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>