drm/i915: Remove use on gma_bus_addr on gen6+

We have enough info to not use the intel_gtt bridge stuff.

v2: Move setup of mappable_base above the legacy init stuff because we
still need that on older platforms. (Daniel)

v3: Remove the dev_priv hunk which was rebased in by accident

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com> (v2)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>