drm/exynos: Clear drvdata after component unbind

When components are unbound, DRM driver is unregistered and freed,
so clear drvdata to avoid potential use-after-free issue in
suspend/resume paths.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>