drm/amd/display: Use SIGNAL_TYPE_NONE in disable_output unless eDP

drm/amd/display: Use SIGNAL_TYPE_NONE in disable_output unless eDP

[WHY]
Currently made a change where disable_output is called using signal_type.
Using actual signal_type when calilng disable_output in power_down_encoders
would make DP to HDMI dongle not light up on boot. As it would have signal_type
SIGNAL_TYPE_DISPLAY_PORT.

[HOW]
Set signal_type to SIGNAL_TYPE_NONE unless it is eDP.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>