drm/msm/dsi-staging: validate dsi command size before sending command.

Check if dsi commands size is less than 4096 bytes before sending
the commands. This will ensure we will not run into out of bound
memory access.

Change-Id: I083aaa0d3b37b6009f7295113a5b2f1c9bfdb09a
Signed-off-by: Vara Reddy <varar@codeaurora.org>