drm/nouveau/drm/ttm: Remove set but not used variable 'mem'

drivers/gpu/drm/nouveau/nouveau_ttm.c: In function nouveau_vram_manager_new:
drivers/gpu/drm/nouveau/nouveau_ttm.c:66:22: warning: variable mem set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/nouveau/nouveau_ttm.c: In function nouveau_gart_manager_new:
drivers/gpu/drm/nouveau/nouveau_ttm.c:106:22: warning: variable mem set but not used [-Wunused-but-set-variable]

They are not used any more, so remove it.

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