agp/intel: Add gma_bus_addr

It is no longer used in the i915 code, so isolate it from the shared
struct.

This was originally part of:
commit 0e275518f325418d559c05327775bff894b237f7
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Mon Jan 14 13:35:33 2013 -0800

    agp/intel: decouple more of the agp-i915 sharing

    Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
    Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
That commit had some other hunks which can't be used due to issues
Daniel found in previous commits.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: drop squash notice from the commit since it's imo ok to keep
this one separate.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>