debian: increase the filesystem size

A bit more space is needed when we copy the ddk files using scp, as we
need to store both ddk files and the compressed version of them.
Allocating 4G instead of 2G, so both compressed and
decompressed mali files fit in the allocated MALI_SIZE.

Unit test framework disabled:
892M    mali.tar.xz
2.5G    mali

Unit test framework enabled:
2.1G    mali.tar.xz
5.5G    mali

debian base size - 816M

Signed-off-by: Alex Dobrescu <alex.dobrescu@arm.com>