drm/exynos: remove unused exynos_crtc->win_enable() callback

None of the exynos crtc drivers implements win_enable() so remove it for
better clarity of the code.

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

diff --cc drivers/gpu/drm/exynos/exynos_drm_drv.h
Simple merge