The modeset lock acquired in dp_mst_connector_detect is not
being released after the detection is completed, which
causes subsequent deadlock.
Added a call to drm_modeset_drop_locks when coming out of
dp_mst_connector_detect.
Change-Id: Ibce79934d0ac23fed4985291c5c6078cb982e812
Signed-off-by: Sudarsan Ramesh <sudarame@codeaurora.org>