drm/exynos: add default pixel format for plane

DRM checks whether the plane supports a pixel format of fb when plane is
updated. This adds a default pixel format supporting format exynos drm
plane.

Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>