drm/i915: Remove gtt_mappable_total

With the assertion from the previous patch in place, it should be safe
to get rid gtt_mappable_total. Keeps things saner to not have to track
the same info in two places.

In order to keep the diff as simple as possible and keep with the
existing gtt_setup semantics we opt to keep gtt_mappable_end. It's not
as consistent with the 'total' used in the previous patch, but that can
be fixed later.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[Ben modified commit message]
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -259,7 +259,8 @@ static int i915_gem_object_info(struct seq_file *m, void* data)
 		   count, size);
 
 	seq_printf(m, "%zu [%zu] gtt total\n",
-		   dev_priv->mm.gtt_total, dev_priv->mm.mappable_gtt_total);
+		   dev_priv->mm.gtt_total,
+		   dev_priv->mm.gtt_mappable_end - dev_priv->mm.gtt_start);
 
 	mutex_unlock(&dev->struct_mutex);