msm: mdss: Add sanity check for Gamut LUT size

The Gamut LUT size passed from user space needs to go through
a sanity check to avoid heap overflow. This patch adds the missing
sanity check in the Gamut LUT config write path.

Change-Id: I365938e06dbc6ca01961c9be01db10a5a9c863e4
Signed-off-by: Ping Li <pingli@codeaurora.org>