msm: mdss: fix possible out-of-bounds and overflow issue in mdp debugfs

There are few cases where the count argument passed by the user
space is not validated, which can potentially lead to out of bounds
or overflow issues. In some cases, kernel might copy more data than
what is requested. Add necessary checks to avoid such cases.

Change-Id: Ifa42fbd475665a0ca581c907ce5432584ea0e7ed
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>