drm/exynos: hdmi: Fix potential NULL pointer dereference error

This is an unlikely case. However to silence the following smatch error
add a NULL check:
drivers/gpu/drm/exynos/exynos_hdmi.c:2486 hdmi_probe()
error: potential NULL dereference 'match'.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>