disp: msm: sde: use proper API to retrieve connector state in check phase

Use sde_connector_state_get_topology API instead of
drm_atomic_get_connector_state since the latter can
return a blank connector state if not found in
global atomic state.

Change-Id: Ib5b58c6cab438a367a885308341d6bccc624f777
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>