disp: msm: dp: add enc availability check in mode_valid

Add encoder availability check in mode_valid. This is a rollback in
commit 2446602565ec ("drm/msm/dp: add private state to dp_mst_bridge")
where active_enc_cnt is removed.

Change-Id: Ia6d4aaef53b537dbe641919496bbd351c3099a85
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>