drm/exynos: init kms poll after creation of connectors

KMS poll init helper should be run when connectors are created,
otherwise it will not schedule connection detector.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>