gpu: ion: Modify zeroing code so it only allocates address space once

vmap/vunmap spend a significant amount of time allocating the
address space to map into.  Rather than allocating address space
for each page, instead allocate once for the entire allocation
and then just map and unmap each page into that address space.

Change-Id: I59fe2c0c5d0b43f5197bba7570157dc3949c8d88
Signed-off-by: Rebecca Schultz Zavin <rschultz@google.com>
Git-commit: f5a3b2b65a6c0a620059144719792e55255b29d9
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>