drm/msm/dsi-staging: Add length check before copying to user space

Add a check to make sure that the length of bytes copied to
the destination buffer doesn't exceed the requested buffer
count before calling the copy_to_user to avoid buffer overflow.

Change-Id: Ice834a9e024b09f3b6041399ff55dec00e75b9ef
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>