drm/exynos: remove chained calls to enable

With atomic modesetting all the control for CRTC, Planes, Encoders and
Connectors should come from DRM core, so the driver is not allowed to
enable or disable planes from inside the crtc_enable()/disable() call.

But it needs to disable planes with crtc_disable in exynos driver
internally. Because crtc is disabled before plane is disabled, it means
plane_disable just returns without any register changes, then we cannot
be sure setting register to disable plane when crtc is disable.

This patch removes this chainned calls to enable plane from exynos hw
drivers code letting only DRM core touch planes except to disable plane.
Also it leads eliminable enabled and resume of struct exynos_drm_plane.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>