msm: mdss: Fix for wrong length in copy_to_user

The caller could have a small buf passed (less then < blen).
Since, the length of count and blen is not checked, it can
write beyond the end of buf.

Change-Id: I9138cd742b6166937f3cc1cbf1af36f280c94bdb
Signed-off-by: Rashi Bindra <rbindra@codeaurora.org>