drm/amd/display: Remove unnecessary NULL check in set_preferred_link_settings

[Why]
link_stream is never NULL here as we've dereferenced it a couple lines before
and have done so for a couple months now.

[How]
- Drop the NULL check.
- Initialize where we know link_stream is non-NULL

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>