ion_page_pool: Remove GFP_COMP from page allocation mask

Using GFP_COMP to create compound pages prevents split_pages from being
used to split an order > 0 page into order 0 pages.

Required for future changes to the system_heap allocation algorithm.

Change-Id: I5ab82f717bb003e074124e1ffae3f112c405bc2b
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>