drm/amd/display: Don't try AUX transactions on disconnected link

[Why]
Setting link DPMS off in response to HPD disconnect creates AUX
transactions on a link that is supposed to be disconnected. This can
cause issues in some cases when the sink re-asserts HPD and expects
source to re-enable the link.

[How]
Avoid AUX transactions on disconnected link.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>