[media] mtk-vcodec: fix compiler warning

mtk-vcodec/venc_vpu_if.c:40:30: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  struct venc_vpu_inst *vpu = (struct venc_vpu_inst *)msg->venc_inst;
                              ^

Note: venc_inst is u64.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>