drm/amd/display: Fix some checkpatch.pl errors and warnings in dc_link_dp.c

[Why]
Any Linux kernel code should pass checkpatch.pl with no errors and
little, if any, warning.

[How]
Fixing some spacing errors and warnings.

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