drm/mediatek: fix null pointer dereference

The probe function requests the interrupt before initializing
the ddp component. Which leads to a null pointer dereference at boot.
Fix this by requesting the interrput after all components got
initialized properly.

Fixes: 119f5173

 ("drm/mediatek: Add DRM Driver for Mediatek SoC
MT8173.")
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Change-Id: I57193a7ab554dfb37c35a455900689333adf511c