gpu: ion: replace __GFP_ZERO with manual zero'ing

As a performance optimization, omit the __GFP_ZERO flag when
allocating individual pages and, instead, zero out all of the pages in
one fell swoop.

Change-Id: Ieb9a895d8792727a8a40b1e27cb1bbeae098f581
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>