drm: bridge: mtk_dsi: Switch to devm_drm_of_get_bridge

devm_drm_of_get_bridge is capable of looking up the downstream
bridge and panel and trying to add a panel bridge if the panel
is found.

Replace explicit finding calls with devm_drm_of_get_bridge.

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331154503.66054-3-jagan@amarulasolutions.com