drm/nouveau/instmem: fix uninitialized_var.cocci warning

Fix following coccicheck warning:
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/nv50.c:316:11-12:
WARNING this kind of initialization is deprecated.

`void *map = map` has the same form of
uninitialized_var() macro. I remove the redundant assignement. It has
been tested with gcc (Debian 8.3.0-6) 8.3.0.

The patch which removed uninitialized_var() is:
https://lore.kernel.org/all/20121028102007.GA7547@gmail.com/
And there is very few "/* GCC */" comments in the Linux kernel code now.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228142352.18006-1-guozhengkui@vivo.com