drm/i915: make unsupported fb modifier message DRM_DEBUG

Or users can just spam the log all they want.

Issue introduced in

commit 9a8f0a12
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Fri Feb 27 11:15:24 2015 +0000

    drm/i915/skl: Allow Y (and Yf) frame buffer creation

References: https://bugs.freedesktop.org/show_bug.cgi?id=89628Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -13006,8 +13006,8 @@ static int intel_framebuffer_init(struct drm_device *dev,
 	case I915_FORMAT_MOD_X_TILED:
 		break;
 	default:
-		DRM_ERROR("Unsupported fb modifier 0x%llx!\n",
-				mode_cmd->modifier[0]);
+		DRM_DEBUG("Unsupported fb modifier 0x%llx!\n",
+			  mode_cmd->modifier[0]);
 		return -EINVAL;
 	}