drm/nouveau: Constify struct nouveau_job_ops

"struct nouveau_job_ops" is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

In order to do it, "struct nouveau_job" and "struct nouveau_job_args" also
need to be adjusted to this new const qualifier.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   5570	    152	      0	   5722	   165a	drivers/gpu/drm/nouveau/nouveau_exec.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   5630	    112	      0	   5742	   166e	drivers/gpu/drm/nouveau/nouveau_exec.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/860e9753d7867aa46b003bb3d0497f1b04065b24.1718381285.git.christophe.jaillet@wanadoo.fr