drm/amd/display: take signal type from link

[Why]
Signal is update to EDP when driver disable first encoder. The
following encoder using SIGNAL_TYPE_EDP to handle other
device. When encoder signal is HDMI, driver will detect it is dp
and release phy. It cause hw hang.

[How]
Take signal type from link->connector_signal.

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>