drm/nouveau/dmem: remove set but not used variable 'drm'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/nouveau/nouveau_dmem.c: In function 'nouveau_dmem_free':
drivers/gpu/drm/nouveau/nouveau_dmem.c:103:22: warning:
 variable 'drm' set but not used [-Wunused-but-set-variable]
  struct nouveau_drm *drm;
                      ^

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>