drm/amd/display: fix NULL dereference

The following patch will fix a minor issue where a debug message is
referencing an struct that has just being checked whether is null or
not. This has been noticed by using coccinelle, in the following output:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c:540:25-29: ERROR: aconnector is NULL but dereferenced.

Fixes: 5d72e247 ("drm/amd/display: switch DC over to the new DRM logging macros")
Signed-off-by: José Pekkarinen <jose.pekkarinen@foxhound.fi>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>