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: d53e8a7e (linux-purism-librem5: enable CONFIG_UDMABUF (MR 5584))