{linux-postmarketos-allwinner,linux-purism-librem5}: restore CMA size

This partly reverts the commit mentioned below. When disabling CMA I thought that libcamera was the only user of it, which is only true for userspace, though. In fact CMA is used and often even required by the kernel for HW video decoders, display engines and other hardware that might not have a MMU.

Thus restore the previously established value.

Fixes: 196ece02 (linux-postmarketos-allwinner: enable CONFIG_UDMABUF (MR 5585)) Fixes: d53e8a7e (linux-purism-librem5: enable CONFIG_UDMABUF (MR 5584))

Sorry for not researching this directly - TIL a lot :) Note that this might explain issues we're seeing on other devices as well. Values like 256MB will often not be enough to allocate the required number of buffers by modern codecs for 4K@60FPS - and a bunch of other devices don't have CMA enabled in the first place. But that need further investigations.