drm/amd/display: Add new case to get spread spectrum info

[WHY]
New minor revision needs to be handled

[HOW]
Add switch case and assert to catch missing switch cases in the future

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>