drm: Allow userspace to ask for universal plane list (v2)

Userspace clients which wish to receive all DRM planes (primary and
cursor planes in addition to the traditional overlay planes) may set the
DRM_CLIENT_CAP_UNIVERSAL_PLANES capability.

v2: Hide behind drm.universal_planes module option [suggested by
    Daniel Vetter]

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/drm_ioctl.c

@@ -328,6 +328,13 @@ drm_setclientcap(struct drm_device *dev, void *data, struct drm_file *file_priv)
 			return -EINVAL;
 		file_priv->stereo_allowed = req->value;
 		break;
+	case DRM_CLIENT_CAP_UNIVERSAL_PLANES:
+		if (!drm_universal_planes)
+			return -EINVAL;
+		if (req->value > 1)
+			return -EINVAL;
+		file_priv->universal_planes = req->value;
+		break;
 	default:
 		return -EINVAL;
 	}