drm/exynos: change file license to GPL

This patch changes file license to GPL

Most of exynos files had been copied from some random
file and not updated correctly. So this patch corrects
the file license.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>