drm/exynos: Add create_connector callback

This creates a new display hook called create_connector. The purpose is
to allow the display driver to create its own drm_connector instead of
using the exynos_drm_connector. This moves things closer to completely
removing the exynos_drm_connector abstraction.

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