msm: mdss: sanitize histogram bin count

Ensure that histogram bin count value is sanitized,
this ensures that the length of data copied back to
user remains in check. This prevents excessive
data in kernel memory from being copied back to user.

Change-Id: I0f7787603bad950c0b187d8c38d4fa3366bc06b6
CRs-Fixed: 517476
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>