drm/exynos: check for null pointers in error handling

Smatch error from arm build: drivers/gpu/drm/exynos/
exynos_hdmi.c:2374 hdmi_probe() error: potential NULL
dereference 'hdata->hdmiphy_port'.

Added check for hdata->hdmiphy_port that it is not NULL.

Signed-off-by: Paul Taysom <taysom@chromium.org>
Signed-off-by: Rahul Sharma <Rahul.Sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>