drm/i915: tune down Y tiling scanout warning

Userspace can easily hit this and does since Ville added a new evil
igt testcase in:

commit 069e35e0fc3785faa562adcfd2dd7bbed4cb1dea
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Mon Mar 4 15:34:06 2013 +0200

    kms_flip: Add flip-vs-bad-tiling test

v2: Fix the spelling in the added comment (Chris).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=63246
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2002,8 +2002,10 @@ intel_pin_and_fence_fb_obj(struct drm_device *dev,
 		alignment = 0;
 		break;
 	case I915_TILING_Y:
-		/* FIXME: Is this true? */
-		DRM_ERROR("Y tiled not allowed for scan out buffers\n");
+		/* Despite that we check this in framebuffer_init userspace can
+		 * screw us over and change the tiling after the fact. Only
+		 * pinned buffers can't change their tiling. */
+		DRM_DEBUG_DRIVER("Y tiled not allowed for scan out buffers\n");
 		return -EINVAL;
 	default:
 		BUG();