CVE-2024-44955 drm/amd/display: Don't refer to dc_sink in is_dsc_need_re_compute

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Don’t refer to dc_sink in is_dsc_need_re_compute

[Why]
When unplug one of monitors connected after mst hub, encounter null pointer dereference.

It’s due to dc_sink get released immediately in early_unregister() or detect_ctx(). When
commit new state which directly referring to info stored in dc_sink will cause null pointer
dereference.

[how]
Remove redundant checking condition. Relevant condition should already be covered by checking
if dsc_aux is null or not. Also reset dsc_aux to NULL when the connector is disconnected.