drm/amd/display: Support external tunneling feature

[Why & How]
The original code only supports the tunneling for embedded one.
To support external tunneling feature, it needs to check
Tunneling_Support bit register.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Jun Lei <jun.lei@amd.com>
Signed-off-by: Cruise Hung <Cruise.Hung@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>