drm/amd/display: Make atomic-check validate underscan changes

When the underscan state was changed, atomic-check was triggering a
validation but passing the old underscan values.  This change adds a
somewhat hacky check in dm_update_crtcs_state that will update the
stream if old and newunderscan values are different.
This was causing 4k on Fiji to allow underscan when it wasn't permitted.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>