mali_mgm: Change default Make target from 'all' to 'modules'

Out-of-tree modules should use 'modules' as their Make target instead
of 'all' to prevent the build system trying to build vmlinux.

Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: I4efdac8a5f6da3b5b5cf8c5c301b30f6836b78bc