drm: exynos: dsi: Use child panel or bridge find helpers

drm: exynos: dsi: Use child panel or bridge find helpers

commit <711c7adc4687> ("drm: exynos: dsi: Use drm panel_bridge API")
added devm_drm_of_get_bridge for looking up if child node has panel
or bridge.

However commit <b089c0a9b14c> ("Revert "drm: of: Lookup if child node
has panel or bridge") has reverted panel or bridge child node lookup
from devm_drm_of_get_bridge which eventually failed to find the DSI
devices in exynos drm dsi driver.

So, use the conventional child panel bridge lookup helpers like it
does before.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220428094808.782938-1-jagan@amarulasolutions.com