drivers: video: Add bounds checking in fb_cmap_to_user

Verify that unsigned int value will not become negative before cast to
signed int..

Bug: 31651010
Change-Id: I548a200f678762042617f11100b6966a405a3920
Git-commit:834c0fa0f4ec6a0ae75b8ce718e51cece90dc9cd
Git-repo: https://android.googlesource.com/kernel/msm


Signed-off-by: Yogendra Charya Tangalapally <yogendra@codeaurora.org>