drm/amd/display: Check if link state is valid

The link state is set to false if there is no link and local sink. Even
though the stream state may not change, it is desirable to commit the
new stream when HPD goes low to high.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Co-developed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>