cma: decrease CMA_ALIGNMENT lower limit to 2

On an embedded system with a tiny (1 MiB) CMA area for video memory, and
a simple enough video pipeline, we can decrease the CMA_ALIGNMENT by a
factor of 2 to avoid wasting memory, as all the allocations for video
buffers will be of the exact same size (dictated by the size of the
screen).

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>