drm/msm/sde: assign the right vsync_source for sde top blk

Assign the right vsync_source for SDE top block. SDE top
block uses vsync_source information to configure the
interinal timers for generating TE signal for command mode
panels working in simulator mode. SDE fails to transfer
the pixels for command mode panels without this TE signal.

Change-Id: I34d6ff88eec984a0d4c7ebdbd20b7c5e76b3a7ae
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>