fbdev: use unsigned integers to prevent overflow

Use unsigned integers to make sure there is
no data overflow. Also modify the size
check for zero in order to make sure this
code is not optimized by the compiler.

CRs-Fixed: 1071271
Change-Id: I35d8cbbd5914f82bef80c8f3d874fedb0b7c74c0
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>