drm/amd/display: Enable 8k60hz mode on single display

[Why]

8k60hz compliace test failing because we restrict it single memory
channels. Workaround by not restricting it on single displays.

[How]

Adding an additional check to DCN314 to restrict 8k60hz mode if it has
more than 1 display connected.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Nicholas Susanto <nicholas.susanto@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>