ion: use alloc_pages in system contig heap

There is no reason to use kzalloc, just call alloc_pages directly.
Change the GFP from GFP_KERNEL to include __GFP_HIGH, to allow it
to return contiguous pages from highmem.  virt_to_* functions
aren't valid on highmem pages, so store the struct page * in an
sg_table in buffer->priv_virt like most other heaps, and replace
virt_to_* with page_to_*.

Change-Id: Ida78888b101f080883716e1fa5038dfc4dbabd16
Signed-off-by: Colin Cross <ccross@android.com>