BACKPORT: drm/exynos: use private plane for crtc

BACKPORT: drm/exynos: use private plane for crtc

The crtc can use private plane instead it has overlay struct. It will be
helpful use plane feature from crtc later.

BUG=chromium:222980
TEST=By hand.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
(cherry picked from commit b5d2eb3bd691c0b6869a2013e719a61c595d73a6)

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Change-Id: I3385e40cac26ebb2d71670f71eb1d3f91638437e
Reviewed-on: https://gerrit.chromium.org/gerrit/46956
Reviewed-by: John Sheu <sheu@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>