drm/i915: Fix gen2 mappable calculations

When I refactored the code initially, I forgot that gen2 uses a
different bar for the CPU mappable aperture. The agp-less code knows
nothing of generations less than 5, so we have to expand the gtt_probe
function to include the mappable base and end.

It was originally broken by me:
commit baa09f5f
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Thu Jan 24 13:49:57 2013 -0800

    drm/i915: Add probe and remove to the gtt ops

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>