drm/radeon: use aperture size not vram size for overlap tests

This fixes a problem where the wrong card conflicts with vesafb in my
x2 system.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_fb.c

@@ -245,7 +245,7 @@ static int radeonfb_create(struct radeon_fbdev *rfbdev,
 		goto out_unref;
 	}
 	info->apertures->ranges[0].base = rdev->ddev->mode_config.fb_base;
-	info->apertures->ranges[0].size = rdev->mc.real_vram_size;
+	info->apertures->ranges[0].size = rdev->mc.aper_size;
 
 	info->fix.mmio_start = 0;
 	info->fix.mmio_len = 0;