drm/amd/display: Fix uninitialized variable usage in core_link_ 'read_dpcd() & write_dpcd()' functions

From Srinivasan Shanmugam
b565f41b9f20ed65d73660ffa6d8e495bc839789 in linux-6.6.y/6.6.15
a58371d632ebab9ea63f10893a6b6731196b6f8d in mainline linux

update drm to linux 6.6.12

Thanks to the OpenBSD Foundation for sponsoring this work.