drm/amd/display: fix dereference of pointer aconnector when it is null

Currently pointer aconnector is being dereferenced by the call to
to_dm_connector_state before it is being null checked, this could
lead to a null pointer dereference.  Fix this by checking that
aconnector is null before dereferencing it.

Addresses-Coverity: ("Dereference before null check")
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>