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

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:191:24: warning:
 variable 'bdev' set but not used [-Wunused-but-set-variable]

It's not used any more and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>