drm/msm/dsi-staging: fix kernel memory exposure

The error handling was needlessly calling the user copy function
while passing error code as the length argument. The fix is to return
early after an error occurs.

Change-Id: I634054b9db5f392bb714410e81d7e780b53f62a8
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>