drm/exynos: remove struct exynos_drm_overlay

struct exynos_drm_overlay has no practical advantage nor serves as
important piece of the exynos API design. The only place it was used
was inside the struct exynos_plane which was just causing a extra
access overhead. Users had to access the overlay first and just then
get the plane information it contains.

This patch merges struct exynos_drm_overlay into struct exynos_plane.
It also renames struct exynos_plane to struct exynos_drm_plane.

The rational is to cut one step to access plane information.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>