gpu: ion: Replace vmalloc() with vzalloc()

Use vzalloc() instead of vmalloc() + memset(,0,).

Change-Id: I24a10114a325bf94c4cb0131eaf3dcd2f901e023
Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Reviewed-on: http://git-master/r/75532
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>