drm/exynos: Add plane support with fimd

The exynos fimd supports 5 window overlays. Only one window overlay of
fimd is used by the crtc, so we need plane feature to use the rest
window overlays.

This creates one ioctl exynos specific - DRM_EXYNOS_PLANE_SET_ZPOS, it
is the ioctl to decide for user to assign which window overlay.

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>