drm/msm/dsi-staging: Prevent holding mutex on error path

During an error scenario, the current code is holding mutex
and erroring out. Prevent this by releasing the mutex and
appropriately return a valid error code.

Change-Id: I97b3fe2716cc88904317b0573b4ecaff4ef8c8a3
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>