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: I4a5c850717c80f75506a36b7ec2bcd55857b8dea
Signed-off-by: Rebecca Schultz Zavin <rschultz@google.com>