vga16fb, drm: vga16fb->drm handoff

let vga16fb claim 0xA0000+0x10000 region as its aperture;
drm drivers don't use it, so we have to detect it and kick
vga16fb manually - but only if drm is driving the primary card

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: James Simmons <jsimmons@infradead.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>