disp: msm: dsi: add check before buffer copy

This change adds a check for the length of the buffer
before copying it to avoid a buffer overflow.

Change-Id: I146895660be4060d9896706636257a57c6aef94f
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>
Signed-off-by: Chirag Khurana <ckhurana@codeaurora.org>