drm/amd/display: dc_link: code clean up on enable_link_dp function

Coding style clean up on enable_link_dp function as suggested by
checkpatch.pl:

CHECK: Lines should not end with a '('
WARNING: line over 80 characters
WARNING: suspect code indent for conditional statements (8, 24)
CHECK: braces {} should be used on all arms of this statement
ERROR: else should follow close brace '}'
CHECK: Comparison to NULL could be written
       "link->preferred_training_settings.fec_enable"

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