mm: cma: fix alignment requirements for contiguous regions 

Contiguous Memory Allocator requires each of its regions to be aligned
in such a way that it is possible to change migration type for all
pageblocks holding it and then isolate page of largest possible order from
the buddy allocator (which is MAX_ORDER-1). This patch relaxes alignment
requirements by one order, because MAX_ORDER alignment is not really
needed.

Change-Id: I57798e709636873da27520bc42f7bdb03af15beb
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
CC: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>