drm/nouveau/mmu/gp100: remove unused variable

Fixes a compilation issue introduced because I forgot to test with WERROR
enabled.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: DRI <dri-devel@lists.freedesktop.org>
Cc: nouveau@lists.freedesktop.org
Fixes: 404046cf

 ("drm/nouveau/mmu/gp100-: drop unneeded assignment in the if condition.")
Acked-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012133334.1737918-1-kherbst@redhat.com