Install kernel module sources for drm-current-kmod.

Install kernel module sources and makefiles to
$PREFIX/sys/modules/drm-current-kmod. This permits kernel builds to
build the DRM modules as part of buildkernel via the LOCAL_MODULES
framework. These modules are installed to the kernel's directory and
are then preferred to the generic modules installed in /boot/modules.

The installation of sources is controlled by a new SOURCES option
which defaults to on.

Reviewed by: zeising, manu
Differential Revision: https://reviews.freebsd.org/D20990