drm/amd/display: check link status before disable stream

[Why]
1. Set second screen only then unplug external monitor
2. Enter to S4 then plug in external monitor
3. Resume from S4, eDP will not turn off when OS set
   second screen only
Sometimes OS will not set eDP power up cause eDP dpms_off
keep true then driver skipp disable stream

[How]
When drvier try to disable stream, add link status condition

Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>