drm/nouveau: Remove unused variable

drm/nouveau: Remove unused variable

Trivial removal of an unused variable. Not sure how it snuck by me and
build bots in the 7c99616e3fe7.

Fixes: 7c99616e3fe7 ("drm: Remove drm_mode_config::fb_base")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thomas Zimmermann <tzimemrmann@suse.de>
Cc: Christian König <christian.koenig@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221021010703.536318-1-zack@kde.org