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 baa09f5fd8
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>

drivers/gpu/drm/i915/i915_drv.h

@@ -399,7 +399,8 @@ struct i915_gtt {
 
 	/* global gtt ops */
 	int (*gtt_probe)(struct drm_device *dev, size_t *gtt_total,
-			  size_t *stolen);
+			  size_t *stolen, phys_addr_t *mappable_base,
+			  unsigned long *mappable_end);
 	void (*gtt_remove)(struct drm_device *dev);
 	void (*gtt_clear_range)(struct drm_device *dev,
 				unsigned int first_entry,