drm: fd.o bug #11895: Only add the AGP base to map offset if the caller didn't.

The i830 and newer intel 2D code adds the AGP base to map offsets already,
because it wasn't doing the AGP enable which used to set dev->agp->base.

Credit goes to Zhenyu for finding the issue.

Signed-off-by: Dave Airlie <airlied@linux.ie>