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

Contributors for this file are :
Chris Wilson <chris@chris-wilson.co.uk>
Denis Efremov <efremov@linux.com>
Jani Nikula <jani.nikula@intel.com>
Maxime Ripard <mripard@kernel.org>
Noralf Trønnes <noralf@tronnes.org>
Sam Ravnborg <sam@ravnborg.org>
Thomas Zimmermann <tzimmermann@suse.de>

Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Denis Efremov <efremov@linux.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
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/20200215180911.18299-2-manu@FreeBSD.org