drm/nouveau/kms/nv50-: Remove several set but not used variables "ret" in disp.c

Fixes the following warning when using W=1 to build kernel:

drivers/gpu/drm/nouveau/dispnv50/disp.c: In function ‘nv50_mstm_cleanup’:
drivers/gpu/drm/nouveau/dispnv50/disp.c:1389:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
 1389 |  int ret;
      |      ^~~
drivers/gpu/drm/nouveau/dispnv50/disp.c: In function ‘nv50_mstm_prepare’:
drivers/gpu/drm/nouveau/dispnv50/disp.c:1413:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
 1413 |  int ret;
      |      ^~~

Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/10