drm/amd/display: code cleanup of dc_link file on func dc_link_construct

Removes codestyle issues in dc_link file, on dc_link_construct and
translate_encoder_to_transmitter as suggested by checkpatch.pl.

Types covered:

CHECK: Lines should not end with a '('
WARNING: Missing a blank line after declarations
CHECK: Alignment should match open parenthesis
CHECK: Comparison to NULL could be written
CHECK: Logical continuations should be on the previous line
CHECK: Blank lines aren't necessary after an open brace '{'

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>