MAINTAINERS: update maintainer entry for Exynos DP driver

Recently, Exynos DP driver was moved from drivers/video/exynos/
directory to drivers/gpu/drm/exynos/ directory. So, I update
and add maintainer entry for Exynos DP driver.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>