Bug#911211: Increase default CMA size for armhf kernels

[Priit Laes <plaes@plaes.org>]

Package: linux-image-armmp-lpae
Version: 4.18+98~bpo9+1

Debian Stretch

Currently, the default CMA size for armhf kernels is set to 16M.
CMA allocator is used by display engine for storing framebuffer data.

At least on sunxi platform (specifically OLinuxino-Lime2-eMMC device)
it causes issues with larger displays (namely 1920x1080), because Xorg
fails to start with following cryptic message:

[snip]
[   855.117] (II) UnloadModule: "fbdev"
[   855.117] (II) Unloading fbdev
[   855.117] (II) UnloadSubModule: "fbdevhw"
[   855.117] (II) Unloading fbdevhw
[   855.118] (==) Depth 24 pixmap format is 32 bpp
[   855.118] (EE) 
Fatal server error:
[   855.118] (EE) AddScreen/ScreenInit failed for driver 0
[   855.118] (EE) 
[   855.119] (EE) 
[/snip]

Increasing CMA size to 24MB (it seems that it can be increased only
by 8MB steps) would be fine, because CMA memory utilization with
1920x1080 display is following:

$ cat /proc/meminfo  |grep Cma
CmaTotal:          24576 kB
CmaFree:            7932 kB