drm/exynos: dpi: fix hotplug fail issue

When connector is created, if connector->polled is
DRM_CONNECTOR_POLL_CONNECT then drm_kms_helper_hotplug_event
function isn't called at drm_helper_hpd_irq_event because the
function will be called only in case of DRM_CONNECTOR_POLL_HPD.

So this patch sets always DRM_CONNECTOR_POLL_HPD flag to
connector->polled of parallel panel driver at connector creation.

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