drm/exynos: Implement mode_fixup manager operation

This patch adds a new manager callback for mode_fixup and pipes it
through exynos_drm_crtc. This will allow the manager drivers to
alter the mode during modeset.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>