drm/msm/dsi-staging: use appropriate length to avoid out of bounds access

While attemtping copy from user buffer, use appropriate length
inorder to avoid access out of bounds.

Change-Id: I647f8f505b318bf1c0568547295cd14c9a6fd830
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>