ion: Remove unnecessary dma_sync in ion_page_pool

The MSM ion implementation moved the dma_sync call to
ion_system_heap_allocate()

It was inadvertently added back to
ion_page_pool_alloc_pages()

in commit aacdfd55
("Merge tag 'lsk-v3.18-15.12-android' into 'msm-3.18'")

Timing information for ION_IOC_ALLOC:
without patch:
50MB average: 90.92 std_dev: 1.87 reps: 10
100MB average: 184.52 std_dev: 3.44 reps: 10

with patch:
50MB average: 87.28 std_dev: 1.93 reps: 10
100MB average: 178.56 std_dev: 2.54 reps: 10

CRs-Fixed: 997730
Change-Id: I61029996e59c754b37f45543c4263a100b427867
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>