drm/i915: BUG() if fences are used on unsupported platform

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2649,7 +2649,7 @@ static void i915_gem_write_fence(struct drm_device *dev, int reg,
 	case 4: i965_write_fence_reg(dev, reg, obj); break;
 	case 3: i915_write_fence_reg(dev, reg, obj); break;
 	case 2: i830_write_fence_reg(dev, reg, obj); break;
-	default: break;
+	default: BUG();
 	}
 }