drm/amd/display: Compare v_front_porch when checking if streams are synchronizable

[Why]
If the front porch of the two timings differ, then there may not be
enough time while both streams are in vertical blank to perform a memory
clock change. This can hang the system.

[How]
Check the each streams timing.v_front_porch when determining if the two
streams are synchronizable.

Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>