drm/mediatek: Find the cursor plane instead of hard coding it

The cursor and primary planes were hard coded.
Now search for them for passing to drm_crtc_init_with_planes

Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>