drm/ttm: remove set but not used variable 'driver'

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

drivers/gpu/drm/ttm/ttm_execbuf_util.c: In function 'ttm_eu_fence_buffer_objects':
drivers/gpu/drm/ttm/ttm_execbuf_util.c:190:24: warning:
 variable 'driver' set but not used [-Wunused-but-set-variable]

It not used any more after
commit f2c24b83 ("drm/ttm: flip the switch, and convert to dma_fence")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>