drm/msm/sde: add mode config mutex lock for connector search

Connection enumeration is used by writeback device to find the
writeback connector. Using connector for_each macros requires
lock of mode config mutex.  Add mutex protection for writeback
connector enumeration.

Change-Id: Ib299f1576cb17d30543daa00df95924b51bc862f
Signed-off-by: Alan Kwong <akwong@codeaurora.org>