drm/amd/display: Add dc cap for dp tunneling

[WHAT]
1. add dc cap for dp tunneling
2. add function to get index of host router

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Cruise Hung <cruise.hung@amd.com>
Signed-off-by: Peichen Huang <PeiChen.Huang@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 29e178d13979cf6fdb42c5fe2dfec2da2306c4ad)
Cc: stable@vger.kernel.org