drm/amd/display: fix typos in several function pointer checks

Fix several copypaste mistakes in *_disable_link_output() functions where
an improper function pointer is checked before dereference.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Vitaliy Shevtsov <v.shevtsov@maxima.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>