drm/vmwgfx: Remove set but not used variable 'restart'

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

drivers/gpu/drm/vmwgfx/vmwgfx_cmdbuf.c: In function 'vmw_cmdbuf_work_func':
drivers/gpu/drm/vmwgfx/vmwgfx_cmdbuf.c:514:7: warning:
 variable 'restart' set but not used [-Wunused-but-set-variable]

It not used any more after commit dc366364 ("drm/vmwgfx: Fix multiple
command buffer context use")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Fixes: dc366364 ("drm/vmwgfx: Fix multiple command buffer context use")