media: mediatek: amend vpu_get_plat_device() documentation

media: mediatek: amend vpu_get_plat_device() documentation

Add a comment to the vpu_get_plat_device() documentation to make it
clear that the VPU platform device is returned with an incremented
reference count (which needs to be dropped after use).

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>