drm/exynos: Don't fail if no TE-gpio is defined for DSI driver

TE-gpio is optional and if it is not found then gpiod_get_optional()
returns NULL. In such case the code will continue and try to convert NULL
gpiod to irq what in turn fails. The failure is then propagated and driver
is not registered.

Fix this by returning early from exynos_dsi_register_te_irq() if no
TE-gpio is found.

Fixes: ee6c8b5a ("drm/exynos: Replace legacy gpio interface for gpiod interface")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>