drm/exynos: dp: Fix NULL pointer dereference due uninitialized connector

Commit 3424e3a4 ("drm: bridge: analogix/dp: split exynos dp driver to
bridge directory") split the Exynos DP core driver into a core driver and
a bridge driver for the Analogix chip since that is also used by Rockchip.

But the change introduced a regression causing a NULL pointer dereference
when trying to access an uninitialized connector in the driver .get_modes:

Fix this by instead of having a connector struct for both the Exynos and
Analogix drivers, just use the connector initialized in the bridge driver.

Fixes: 3424e3a4 ("drm: bridge: analogix/dp: split exynos dp driver to bridge directory")

Reported-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>