drm/exynos: changed exynos_drm_display to exynos_drm_display_ops

exynos_drm_display has function pointes so exynos_drm_display_ops is better
to describe.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>