drm/msm/dsi-staging: Fix buffer overflow and Null pointer dereference

This change fixes two issues in DSI driver: fix for Buffer Overflow-
Local Array Index Out of Bounds and fix for Pointer will be dereferenced
after it was positively checked for NULL.

Change-Id: I71741ead2f34e44de73a18c480bff3c07855a885
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>