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

Contributors for this file are :
Gerd Hoffmann <kraxel@redhat.com>
Maxime Ripard <mripard@kernel.org>
Noralf Trønnes <noralf@tronnes.org>

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Emmanuel Vadot <manu@FreeBSD.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200215180911.18299-3-manu@FreeBSD.org