Bug#808338: Enable ASpeed VGA module

["Timothy Pearson" <kb9vqf@pearsoncomputing.net>]

Package: linux-image-4.2.0-1-powerpc64le
Version: 4.2.6-3
Severity: normal

Many server- and workstation-class boards use the ASpeed Technologies BMC
device as the primary VGA output.  Linux supports initialization of this
device via the AST DRM module, however Debian does not build this module,
leaving users of some systems (e.g. OpenPOWER) without a working VGA
output.

Please enable the AST DRM module with CONFIG_DRM_AST=m in the kernel
configuration.