drm/msm/dp: add encoder and connector reservation dp-mst

Currently the dp-mst connector is created and destroyed dynamically,
which requires drm client to handle hot-plug and update the whole
connector list when device is connected or disconnected.

A reservation approach is added to allow connector to be created statically
for mst port in devicetree at system startup stage, and drm client can
control the connector in the same way as non-mst connectors. This optional
devicetree property is useful for system with a built-in mst-hub, and can
simplify the drm client design.

Change-Id: Ied59a5302b3f4cd1553619e6ee5acd7ce62df9ac
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>