drm/mediatek: clean up driver data initialisation

drm/mediatek: clean up driver data initialisation

The platform and drm devices are only used to look up the drm device and
its driver data respectively when initialising the driver data during
bind().

Drop the reference counts as soon as they have been used to make the
code more readable.

Note that the crtc count is never incremented on lookup failures.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250829090345.21075-3-johan@kernel.org/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>