drm/amd/display: Fix null pointer on analog detection

drm/amd/display: Fix null pointer on analog detection

Check if we have an amdgpu_dm_connector->dc_sink first before
adding common modes for analog outputs. If we don't have a
sink yet we can safely skip this.

Fixes: 70181ad96ec2 ("drm/amd/display: Add common modes to analog displays without EDID")
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>