video: tegra: nvmap: Add support for zeroed pages

Add support to alloc zeroed pages for user space alloc requests. Also
define a config option to force userspace allocation requests to be
zeroed.

Change-Id: I75d3b2bc36e808f1470b423578ec4cba99e0f967
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/122549
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>