drm/amd/display: should check error using DC_OK

core_link_read_dpcd returns only DC_OK(1) and DC_ERROR_UNEXPECTED(-1),
the caller should check error using DC_OK instead of checking against 0

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>