drm/exynos: dpi: Change connector type to DPI

When exynos_drm_dpi.c was written, DRM_MODE_CONNECTOR_DPI did not exist
yet and I guess that's the reason why DRM_MODE_CONNECTOR_VGA was used as
the connector type.

However, now it makes more sense to use DRM_MODE_CONNECTOR_DPI as the
connector type.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Inki Dae <inki.dae@samsung.com>