drm/nouveau/disp: sizeof() wrong pointer

"data" is a void pointer and "args" is "data" after we have casted it to
a struct.  We care about the size of the struct here.  Btw,
sizeof(*data) is 1.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>