[Pkg-raspi-maintainers] Bug#980536: CMA=64M insufficient for starting firefox on RPi4B with 2xFullHD

[ibu ☉ radempa]

Hi Gunnar,

on a RPi4B with 8G RAM and two Full HD displays connected I am running 
bookworm. After upgrading firefox (102.15.1esr-1~deb12u1 to 
115.3.0esr-1~deb12u1) recently, it failed to start and threw multiply:

```
DRM_IOCTL_MODE_CREATE_DUMB failed: Cannot allocate memory
Failed to create scanout resource
```

After setting CMA=128M in /etc/default/raspi-firmware, update-initramfs 
and reboot firefox started properly and is usable.

```
# grep -i cma /proc/meminfo
CmaTotal:         131072 kB
CmaFree:           48612 kB
```

And if I run mpv (with CMA=128M) in addition to firefox, I occasionally 
also get the above error.

I suggest removing the notice in /etc/default/raspi-firmware concerning 
RPi4, or referring to this bug, and maybe updating other docs and the 
image when you find time, because I guess firefox is a frequently used 
application on this device and if it fails, it would push off users.

Cheers,
ibu