drm/mediatek: mtk_drm_drv.c: Add of_node_put() before goto

drm/mediatek: mtk_drm_drv.c: Add of_node_put() before goto

Each iteration of for_each_child_of_node puts the previous
node, but in the case of a goto from the middle of the loop, there is
no put, thus causing a memory leak. Hence add an of_node_put before the
goto in two places.
Issue found with Coccinelle.

Fixes: 119f5173628a (drm/mediatek: Add DRM Driver for Mediatek SoC MT8173)
Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>