drm/msm/dsi-staging: fix buffer overflow errors

This change fixes the buffer overflow - array index out of bounds
errors by allocating extra space for buf inorder to NULL terminate it.

Change-Id: I9b3f79d049a7a5f617ac74c5d41c416103a6503a
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>