drm/client: Dual licence the header in GPL-2 and MIT

Source file was dual licenced but the header was omitted, fix that.
Contributors for this file are:
Daniel Vetter <daniel.vetter@ffwll.ch>
Matt Roper <matthew.d.roper@intel.com>
Maxime Ripard <mripard@kernel.org>
Noralf Trønnes <noralf@tronnes.org>
Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Emmanuel Vadot <manu@FreeBSD.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200430153347.85323-1-manu@FreeBSD.org

include/drm/drm_client.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0 or MIT */
 
 #ifndef _DRM_CLIENT_H_
 #define _DRM_CLIENT_H_