drm/exynos: add format list of plane

NV12, NV12M and NV12MT are added to format list of plane to use these
formats for hdmi vp layer.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>